Chemistry · Chapter 20

Organic Chemistry

Carbon is a quiet element with an unusual talent. It makes four bonds, and it bonds to itself without complaint. From that one habit comes a library of millions of compounds. Fuels, plastics, medicines, perfumes and your own cells are all built on carbon frames. This chapter hands you the map. You will meet the families, called functional groups. You will name them with the rules chemists share. You will draw one molecule four different ways. And you will watch a tiny change, like one oxygen moved one spot, turn a gas into a solvent or a sweet smell into vinegar.

By the end of this chapter you can

Draw alkanes as molecular, condensed and skeletal structures.
Name branched alkanes by the IUPAC rules and count structural isomers.
Tell alkanes, alkenes and alkynes apart by formula, shape and reaction.
Explain cis and trans isomers, and say when a molecule can have them.
Show why benzene substitutes instead of adding.
Match each functional group to its family, its name ending and its behavior.
Build addition and condensation polymers from the right monomers.

Section 1Alkanes and isomers

Four bonds, and a talent for chains

Carbon sits in group 14 of the table. It holds four valence electrons. So it forms four bonds, every time. Those four bonds spread to the corners of a tetrahedron. The angle between them is about 109.5°. Carbon also bonds to other carbon atoms with ease. Chains, rings and branches all follow from that. One element earns a whole chapter.

Organic chemistry is the chemistry of carbon compounds. The plainest ones hold carbon and hydrogen only. Those are the hydrocarbons. An alkane is a hydrocarbon with single bonds only. Each carbon carries all the hydrogen it can hold. Chemists call that saturated. The formula follows one pattern, C_nH_2n+2. Six carbons give C₆H₁₄.

The alkanes form a homologous series. Each member adds one CH₂ unit to the one before it. The first four names are old ones: methane, ethane, propane, butane. From five on, a Greek number leads. Boiling points climb as the chain grows, because longer chains cling to each other harder. Methane is a gas you burn for heat. Octane is a liquid you pour into a tank.

Carbons	Name	Formula	Boiling point	Where you meet it
1	methane	CH₄	−161.5 °C	natural gas
2	ethane	C₂H₆	−88.6 °C	natural gas
3	propane	C₃H₈	−42.1 °C	grill tanks
4	butane	C₄H₁₀	−0.5 °C	lighters
5	pentane	C₅H₁₂	36.1 °C	lab solvent
6	hexane	C₆H₁₄	68.7 °C	glue, extraction
8	octane	C₈H₁₈	125.7 °C	gasoline
10	decane	C₁₀H₂₂	174.1 °C	kerosene

Three ways to draw one molecule

A molecular formula counts atoms and stops there. C₄H₁₀ tells you nothing about shape. A condensed structural formula groups the atoms carbon by carbon. Butane becomes CH₃CH₂CH₂CH₃. A skeletal structure is faster still. Each line is a bond. Each corner and each loose end is a carbon. The hydrogens are left out, because you can work them out. A corner with two lines must hold two hydrogens.

Skeletal drawings look lazy. They are not. Hiding the hydrogens lets the frame show, and the frame is where the chemistry happens.

One formula, two molecules. Butane and 2-methylpropane share C₄H₁₀ and boil 11 degrees apart. Below, the numbering rule picks the end nearest the first branch.

Same formula, different molecule

Butane can be built two ways. Four carbons in a row give butane. Three in a row with one hanging off the middle give 2-methylpropane. Both are C₄H₁₀. Both sit in real bottles with real labels. They boil eleven degrees apart. Molecules like this are structural isomers. They share a formula but wire their atoms up differently.

Isomer counts grow fast. C₄H₁₀ has 2 ways to connect. C₅H₁₂ has 3. C₆H₁₄ has 5. C₈H₁₈ has 18. C₁₀H₂₂ has 75. Each new carbon opens more places to put a branch.

Chains can also close into rings. A cycloalkane has the formula C_nH_2n, two hydrogens short of the open chain. Cyclohexane, C₆H₁₂, is the one you will see most. Its carbons still want 109.5°, so the ring folds into a shape called a chair.

Naming by the rules

Seventy-five shapes under one formula need a naming system. The IUPAC rules do that work. Find the longest continuous carbon chain first, and let it set the base name. Number that chain from the end nearest the first branch. Name each branch as an alkyl group, so CH₃ is methyl and C₂H₅ is ethyl. List the branches in alphabetical order with their numbers. Use di, tri and tetra for repeats. Those numbers are called locants, and the right set is the lowest one.

Common names still float around. 2-methylpropane is often called isobutane. 2,2-dimethylpropane is called neopentane. Those names are fine in a shop. The IUPAC name is the one that works everywhere.

Worked example · Naming a branched alkane

Name CH₃CH₂CH(CH₃)CH₂CH₂CH₃.

Step 1 · Find the longest chainCount the carbons written in a row, skipping the one in brackets. That gives 6, so the base name is hexane.

Step 2 · Number it both waysThe branch sits on the third carbon from the left. From the right it is the fourth. Pick 3.

Step 3 · Name the branchThe branch is one carbon, CH₃, so it is a methyl group.

Step 4 · Write the name3-methylhexane.

CheckCarbons: 6 in the chain plus 1 in the branch is 7. For an alkane, H = 2(7) + 2 = 16. Adding the written hydrogens gives 3 + 2 + 1 + 3 + 2 + 2 + 3 = 16. The formula C₇H₁₆ agrees.

Worked example · Burning an alkane

Balance the combustion of octane, C₈H₁₈, in oxygen.

Step 1 · Write the productsA clean burn gives CO₂ and H₂O only.

Step 2 · Carbon first8 carbon atoms need 8 CO₂.

Step 3 · Hydrogen next18 hydrogen atoms need 9 H₂O.

Step 4 · Count oxygen on the right8 × 2 from CO₂ plus 9 × 1 from water is 25 oxygen atoms. That is 12.5 O₂.

Step 5 · Clear the halfDouble every coefficient: 2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O.

CheckCarbon 16 and 16. Hydrogen 36 and 36. Oxygen 50 on the left, and 32 + 18 = 50 on the right. Balanced.

Key points

Carbon forms four bonds at about 109.5°, so chains, rings and branches are all easy.
Alkanes are saturated. Their formula is C_nH_2n+2, and rings are C_nH_2n.
In a skeletal drawing every corner and every free end is a carbon atom.
Structural isomers share a formula but not a skeleton, so they are different substances.
Name from the longest chain, and number toward the nearer branch.
The correct locant set is the lowest one the structure allows.

Interactive Build carbon chains and branch them into isomers. Open full ↑

Section 2Alkenes and alkynes

One bond more, two hydrogens fewer

An alkene holds at least one double bond between carbons. Its formula is C_nH_2n. An alkyne holds a triple bond, and its formula is C_nH_2n-2. Both are unsaturated, which means they carry less hydrogen than the matching alkane. That missing hydrogen is the whole story. The extra bonds are loose electron density, and reagents go straight for it.

Shape follows the bonding. A doubly bonded carbon is flat, with angles near 120°. A triply bonded carbon is a straight line at 180°. A single bond can spin like a wheel on an axle. A double bond cannot spin at all. The two ends are locked into one plane.

Names change at the ending. Alkenes end in -ene and alkynes end in -yne. Number the chain so the first carbon of the multiple bond gets the lowest number. CH₂=CHCH₂CH₃ is 1-butene. CH₃CH=CHCH₃ is 2-butene. Two old names stuck hard. Ethene is still sold as ethylene, and ethyne is nearly always called acetylene.

Those extra bonds make these molecules worth money. Ethene is the most produced organic chemical in the world. It is the feedstock for polyethylene, and it doubles as a plant hormone that ripens fruit. Propene feeds polypropylene the same way. Ethyne burns in oxygen at around 3100 °C, which is hot enough to cut steel. A triple bond packs a lot of energy into one short, strong link.

Bond order, shape and locking. More bonds between two carbons means a shorter, wider angle and no twisting. That lock is what makes cis and trans real. At the bottom, bromine adds across the double bond and nothing is thrown away.

Counting the missing hydrogen

A formula alone can tell you how unsaturated a compound is. Compare its hydrogen count with the saturated count for the same carbons. Each missing pair of hydrogens means one degree of unsaturation. One degree is one double bond or one ring. A triple bond counts as two.

Worked example · Degrees of unsaturation

A hydrocarbon is C₆H₁₀. How unsaturated is it?

Step 1 · Find the saturated countFor 6 carbons, a saturated alkane holds 2(6) + 2 = 14 hydrogens.

Step 2 · Subtract14 − 10 = 4 hydrogen atoms are missing.

Step 3 · Divide by two4 ÷ 2 = 2 degrees of unsaturation.

Step 4 · Read itTwo degrees could be two double bonds, one triple bond, or one ring plus one double bond. The formula cannot tell you which.

CheckCyclohexene is C₆H₁₀, and it has one ring and one double bond. That is 2 degrees, as expected.

Addition keeps every atom

An addition reaction opens the multiple bond and attaches a piece to each carbon. Nothing is thrown away. Hydrogenation adds H₂ over a platinum or nickel catalyst. Halogenation adds Br₂, and the orange color fades as it goes, which makes a fast test for unsaturation. Hydration adds water with an acid catalyst and gives an alcohol. Hydrogen halides add as well. Markovnikov's rule says the hydrogen goes to the carbon that already carries more hydrogen.

Alkanes cannot add, because they have no loose bond to open. They substitute instead. Substitution swaps one atom for another and spits out a second product. Chlorine and butane under light give chlorobutane plus HCl. That HCl is waste. Addition routes every atom into the product you wanted, so they waste far less. Chemists score this with atom economy.

Worked example · Atom economy, addition against substitution

Compare Br₂ adding to 2-butene with Cl₂ substituting on butane.

Step 1 · Addition massesC₄H₈ is 56.11 g/mol and Br₂ is 159.81 g/mol. Together that is 215.92 g/mol.

Step 2 · Addition productC₄H₈Br₂ is 56.11 + 159.81 = 215.92 g/mol. Atom economy = 215.92 ÷ 215.92 = 100%.

Step 3 · Substitution massesC₄H₁₀ is 58.12 g/mol and Cl₂ is 70.90 g/mol. Together that is 129.02 g/mol.

Step 4 · Substitution productC₄H₉Cl is 92.57 g/mol. Atom economy = 92.57 ÷ 129.02 = 0.717, or 71.7%.

CheckThe missing 28.3% leaves as HCl, which is 36.46 g/mol. 36.46 ÷ 129.02 = 0.283. The two shares add to 100%.

Key points

Alkenes are C_nH_2n and alkynes are C_nH_2n-2.
Double bonds give 120° and a flat, locked unit. Triple bonds give 180° and a straight line.
Degrees of unsaturation = (2C + 2 − H) ÷ 2, where C and H are the atom counts.
Cis and trans need two different groups on each carbon of the double bond.
Addition opens a multiple bond and keeps all atoms. Substitution swaps and makes waste.
Markovnikov's rule sends the added hydrogen to the carbon with more hydrogen already.

Interactive Add and remove bonds and compare addition with substitution. Open full ↑

Section 3Aromatic compounds

A ring that refused to behave

Benzene is C₆H₆. Six carbons and six hydrogens, no more. Run the unsaturation count and you get (14 − 6) ÷ 2 = 4 degrees. One ring plus three double bonds fits that number. In 1865 Kekulé drew exactly that, a six-sided ring with single and double bonds taking turns.

Then the tests came back strange. An alkene wipes the color out of bromine water in seconds. Benzene sits there and does nothing. An alkene takes up hydrogen over a catalyst at room temperature. Benzene needs heat, pressure and a hard push. Something was missing from the drawing.

Measurement found it. Every carbon to carbon bond in benzene is the same length, 139 pm. A plain single bond is 154 pm. A plain double bond is 134 pm. Benzene lands between the two, at every bond in the ring. The molecule is a flat, regular hexagon with 120° angles all the way round.

Electrons that belong to the whole ring

The true structure is neither Kekulé drawing. Each ring carbon keeps one electron in a p orbital that points above and below the ring. Those six electrons spread out over all six carbons at once. That spreading is delocalization. The two Kekulé drawings are resonance structures, and the real molecule is a single blend of them. Chemists draw the blend as a hexagon with a circle inside.

Spreading electrons out lowers the energy, and the energy drop is measurable. Adding hydrogen to one double bond in cyclohexene releases 120 kJ/mol. Three separate double bonds should therefore release about 360 kJ/mol. Benzene releases only 208 kJ/mol. It began 152 kJ/mol lower in energy than the drawing predicted. That bonus is the resonance energy. A flat ring that earns a bonus like this is called aromatic. The ring needs 4n + 2 delocalized electrons to qualify, and benzene has 6, with n = 1.

Why benzene is calm. Neither alternating drawing is right. The six electrons sit over the whole ring, every bond comes out at 139 pm, and the ring holds 152 kJ/mol of extra stability.

Worked example · Finding the resonance energy

Use hydrogenation heats to measure benzene's extra stability.

Step 1 · Measure one double bondCyclohexene plus H₂ gives cyclohexane and releases 120 kJ/mol.

Step 2 · Predict three of them3 × 120 kJ/mol = 360 kJ/mol, if the three bonds in benzene were ordinary and separate.

Step 3 · Measure benzeneBenzene plus 3 H₂ gives cyclohexane and releases only 208 kJ/mol.

Step 4 · Take the difference360 − 208 = 152 kJ/mol. Benzene sat 152 kJ/mol lower in energy before the reaction started.

Check1,3-cyclohexadiene releases 232 kJ/mol, just under the 240 kJ/mol predicted for two bonds. Two neighboring bonds give a tiny bonus. A full ring gives a huge one.

Substitution keeps the prize

Addition would break the delocalized ring apart. The product would hand back that 152 kJ/mol, so addition is a bad trade. Substitution is not. Swap one hydrogen for a new group and the ring stays whole and aromatic. So benzene reacts by electrophilic aromatic substitution. Bromine with an FeBr₃ catalyst gives bromobenzene plus HBr. Nitric acid with sulfuric acid gives nitrobenzene plus water. Hot concentrated sulfuric acid gives benzenesulfonic acid. In each case one hydrogen walks out and the ring survives.

Names for rings mix old and new. Methylbenzene is almost always called toluene. A ring with OH is phenol, and a ring with NH₂ is aniline. When the ring itself is the branch, it is a phenyl group. Two groups on one ring can sit in three patterns. The 1,2 pattern is ortho, 1,3 is meta and 1,4 is para. The three dimethylbenzenes are known as the xylenes.

Fuse two rings and you get naphthalene, the old mothball smell. Fuse more and you reach the ring systems in soot and tar. Those big flat rings are stable, which is exactly why they stick around in the environment for so long.

Key points

Benzene is C₆H₆, flat, with 120° angles and six equal 139 pm bonds.
Six p electrons spread over all six carbons. That is delocalization.
The two Kekulé drawings are resonance forms, not two real substances.
Delocalization buys 152 kJ/mol of extra stability, called the resonance energy.
Aromatic rings are flat and hold 4n + 2 delocalized electrons.
Benzene substitutes rather than adds, because substitution keeps the ring aromatic.

Interactive Delocalize benzene's electrons and see why it is stable. Open full ↑

Section 4Alcohols and ethers

The group makes the family

A hydrocarbon is a frame and little more. Swap one hydrogen for something else and the molecule gains a personality. A functional group is that reactive piece. It sets the boiling point, the solubility and the reactions. The carbon frame mostly just comes along for the ride. Chemists write R for the frame so they can watch the group alone.

Alcohols carry a hydroxyl group

An alcohol holds a hydroxyl group, written –OH, on a carbon. To name one, drop the final -e from the alkane name and add -ol. Methane becomes methanol. Propane becomes 1-propanol or 2-propanol, depending on where the OH sits. 2-propanol is the rubbing alcohol in the cabinet. You can also class an alcohol by the carbon that holds the OH. One other carbon on it makes the alcohol primary, two makes it secondary and three makes it tertiary.

That small group changes everything. Oxygen pulls electrons much harder than hydrogen, so the O–H bond is very polar. The hydrogen on one molecule is then drawn to the oxygen on the next. That attraction is a hydrogen bond, and it takes real heat to pull apart. Watch three molecules of nearly equal mass. Propane boils at −42.1 °C. Dimethyl ether boils at −24.8 °C. Ethanol boils at 78.3 °C. Only ethanol has an O–H bond, and it boils over a hundred degrees higher.

Same mass, different grip. Propane has nothing polar to offer. The ether can accept a hydrogen bond from water but cannot donate one. Ethanol does both, so it boils far higher and mixes with water freely.

Worked example · Two molecules, one molar mass

Show that ethanol and dimethyl ether have the same molar mass, then explain the boiling gap.

Step 1 · Write the shared formulaEthanol is CH₃CH₂OH and dimethyl ether is CH₃OCH₃. Both are C₂H₆O.

Step 2 · Add the massesC: 2 × 12.011 = 24.022 g/mol. H: 6 × 1.008 = 6.048 g/mol. O: 1 × 15.999 = 15.999 g/mol.

Step 3 · Total24.022 + 6.048 + 15.999 = 46.069, so 46.07 g/mol for both.

Step 4 · Compare boiling points78.3 − (−24.8) = 103.1 °C apart. Mass cannot explain that, because the masses match.

CheckOnly ethanol has an O–H bond, so only ethanol can donate a hydrogen bond to its neighbor. Propane, at 44.10 g/mol, boils lower still at −42.1 °C. The group is doing the work, not the mass.

Mixing with water, and running out of patience

Solubility follows the same logic. The OH end is at home in water. The carbon chain is not. Short alcohols win that argument. Methanol, ethanol and 1-propanol mix with water in any amount at all. 1-butanol only dissolves to 7.9 g per 100 mL. 1-hexanol manages 0.6 g. 1-octanol is down to 0.05 g. Add more OH groups and the trend reverses. Ethylene glycol and glycerol each hold several hydroxyls, and both are syrups that mix with water without limit.

Ethers hide the oxygen in the middle

An ether has oxygen sitting between two carbon frames, R–O–R'. Diethyl ether is the classic one. There is no O–H bond here, so an ether cannot donate a hydrogen bond. It can still accept one from water, so small ethers dissolve a little. Diethyl ether holds 6.9 g per 100 mL of water. Boiling points sit far below the matching alcohol. Diethyl ether boils at 34.6 °C and 1-butanol boils at 117.7 °C. Both are C₄H₁₀O.

Alcohols also do reactions an ether cannot. Heat an alcohol with sulfuric acid and water is pulled out, leaving an alkene. That is a dehydration. Two alcohol molecules can instead join and lose one water to make an ether. Oxidation works on alcohols as well. A primary alcohol gives an aldehyde, then a carboxylic acid. A secondary alcohol gives a ketone. A tertiary alcohol has no hydrogen left on that carbon, so it simply refuses.

Key points

A functional group, not the carbon frame, decides how a molecule behaves.
Alcohols are R–OH and take the -ol ending. Count carbons on that one carbon to call it 1°, 2° or 3°.
An O–H bond lets a molecule donate a hydrogen bond, which raises the boiling point sharply.
Ethers are R–O–R'. They accept hydrogen bonds but cannot donate any.
Short alcohols mix with water. Longer chains and fewer OH groups mean less mixing.
Primary alcohols oxidize twice, secondary alcohols once, and tertiary alcohols not at all.

Interactive Spot the functional group and predict boiling point and solubility. Open full ↑

Section 5Aldehydes, ketones, acids and esters

One group, four families

A carbonyl group is a carbon double bonded to oxygen, written C=O. It is strongly polar, because oxygen drags the shared electrons toward itself. That leaves the carbon a little positive and open to attack. What sits next to the carbonyl carbon decides which family you are in.

In an aldehyde the carbonyl carbon also holds a hydrogen, and it sits at the end of the chain. Names end in -al. Methanal is sold as formaldehyde. Ethanal is better known as acetaldehyde. In a ketone the carbonyl carbon holds carbon on both sides. Names end in -one, with a number when the chain is long enough to need one. Propanone is acetone, the nail polish remover. 2-butanone is a paint solvent.

A carboxylic acid puts an OH right on the carbonyl carbon, written –COOH. Names end in -oic acid. Methanoic acid is formic acid, the sting in an ant bite. Ethanoic acid is acetic acid, which is what makes vinegar sour. These are weak acids. Acetic acid has K_a = 1.8 × 10^-5, so its pK_a is 4.74. In a 0.10 M solution only about one molecule in eighty gives up its proton.

An ester replaces the acid's OH hydrogen with a carbon group, giving R–COO–R'. Its name takes two words. The alcohol part comes first as an alkyl name. The acid part follows with an -oate ending. Ethanol plus ethanoic acid gives ethyl ethanoate, usually called ethyl acetate. Esters smell good, which is why flavor labs love them. Methyl butanoate smells like apple. Pentyl ethanoate smells like banana.

Four families, one group. Each structure holds the same C=O unit. Only the neighbors change, and that is enough to change the name, the smell and the chemistry.

Worked example · The pH of vinegar-strength acid

Find the pH of 0.10 M ethanoic acid. K_a = 1.8 × 10^-5.

Step 1 · Write the equilibriumCH₃COOH ⇌ H⁺ + CH₃COO⁻.

Step 2 · Set up the expressionLet x be the H⁺ made. Then K_a = x² ÷ (0.10 − x) = 1.8 × 10^-5.

Step 3 · Assume x is smallx² = (1.8 × 10^-5)(0.10) = 1.8 × 10^-6. So x = 1.3 × 10^-3 M.

Step 4 · Take the logarithmpH = −log(1.34 × 10^-3) = 2.87.

Checkx ÷ 0.10 = 1.3%, which is under 5%, so dropping the x was fair. A pH near 3 matches a bottle of vinegar, which is about this strength.

Making and breaking an ester

Esterification joins an acid and an alcohol with an acid catalyst. Water is the second product, so this is a condensation reaction. The reaction is reversible and settles at equilibrium, so chemists remove the water to push it forward. Run it backward with extra water and the ester splits apart, which is hydrolysis. Boil an ester with sodium hydroxide instead and you get a salt plus the alcohol. Do that to a fat and the salt is soap, a reaction called saponification.

The oxidation ladder

These families are rungs on one ladder. Start at a primary alcohol. Oxidize once and you reach the aldehyde. Oxidize again and you reach the carboxylic acid. 1-propanol becomes propanal, then propanoic acid. A secondary alcohol climbs one rung and stops at the ketone. The ketone carbon has no hydrogen left to lose, so the ladder ends there. Aldehydes sit in the middle and oxidize so readily that an open bottle slowly turns acidic.

Family	Group	Ending	Example	Behavior to remember
Aldehyde	–CHO	-al	ethanal	oxidizes easily to the acid
Ketone	–CO–	-one	propanone	end of the oxidation ladder
Carboxylic acid	–COOH	-oic acid	ethanoic acid	weak acid, pK_a near 4 to 5
Ester	–COO–	alkyl -oate	ethyl ethanoate	made by condensation, split by hydrolysis

Key points

A carbonyl is C=O. Its neighbors decide the family.
Aldehydes end in -al and sit at a chain end. Ketones end in -one and sit inside the chain.
Carboxylic acids are –COOH and are weak acids, with pK_a values near 4 to 5.
Esters are named alcohol part first, then the acid part with -oate.
Acid plus alcohol gives ester plus water. Adding water back reverses it.
Primary alcohol → aldehyde → acid. A secondary alcohol stops at the ketone.

Interactive Compare the carbonyl families and their reactions. Open full ↑

Section 6Amines, amides and polymers

Nitrogen joins the frame

An amine is ammonia with carbon groups in place of hydrogen. Count the carbons bonded to the nitrogen to class it. One carbon makes it primary, two makes it secondary, three makes it tertiary. The simplest one is CH₃NH₂, named methanamine and usually called methylamine. Small amines smell like old fish, and that is not a coincidence. Fish spoilage releases them.

The nitrogen keeps a lone pair of electrons, so an amine is a weak base. It pulls a proton off water and leaves OH⁻ behind. Methylamine has K_b = 4.4 × 10^-4. Ammonia is weaker at 1.8 × 10^-5. Aniline, which is an amine on a benzene ring, is far weaker at 4.3 × 10^-10. The ring drags that lone pair into its delocalized cloud, so the pair is no longer free to grab a proton.

An amide sets nitrogen right beside a carbonyl, giving the C(=O)N unit. An acid and an amine join and lose water to build it. Ethanamide, also called acetamide, is the simplest one. Here too the nitrogen's lone pair is shared with the carbonyl, so an amide is not a base in water. The amide link is flat, strong and hard to break. In proteins that same link is called a peptide bond.

Worked example · The pH of a weak base

Find the pH of 0.10 M methylamine. K_b = 4.4 × 10^-4.

Step 1 · Write the equilibriumCH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻.

Step 2 · Try the shortcutx² = (4.4 × 10^-4)(0.10) = 4.4 × 10^-5, giving x = 6.6 × 10^-3 M. That is 6.6% of 0.10 M, over the 5% limit, so solve it properly.

Step 3 · Solve the quadraticx² + (4.4 × 10^-4)x − 4.4 × 10^-5 = 0 gives x = 6.42 × 10^-3 M of OH⁻.

Step 4 · Convert to pHpOH = −log(6.42 × 10^-3) = 2.19. pH = 14.00 − 2.19 = 11.81.

CheckA pH well above 7 is what a base should give. Ammonia at the same concentration reaches about pH 11.1, a little lower, because its K_b is smaller.

Monomers into chains

A polymer is one very long molecule built from small repeating pieces. The small starting molecule is a monomer. The piece that repeats along the chain is the repeat unit. The number of repeats is the degree of polymerization. Multiply the repeat unit mass by that number and you have the chain's molar mass. Real samples hold chains of many lengths, so the mass quoted is always an average.

An addition polymer grows from alkenes. The double bond opens and the carbons link up, one monomer at a time. Nothing leaves. Ethene gives polyethylene for bags and bottles. Propene gives polypropylene for carpet and caps. Chloroethene gives PVC for pipe. Tetrafluoroethene gives PTFE for nonstick pans. Because nothing is lost, the repeat unit has exactly the same formula as the monomer.

A condensation polymer drops a small molecule, usually water, at every link. Each monomer needs two reactive ends so the chain can keep going both ways. A diacid plus a diamine gives a polyamide. Hexanedioic acid with 1,6-hexanediamine gives nylon-6,6. A diacid plus a diol gives a polyester. Terephthalic acid with ethylene glycol gives PET, the plastic in drink bottles. Amino acids join by the same trick and give proteins, which is why a protein is a natural polyamide.

Two ways to grow a chain. Addition opens double bonds and keeps every atom, so the repeat unit matches the monomer. Condensation joins reactive ends and spits out water, so the repeat unit is lighter than the monomers that made it.

Worked example · How long is a PET chain?

A PET sample averages 38,400 g/mol. Its repeat unit is C₁₀H₈O₄. Find the degree of polymerization.

Step 1 · Mass of the repeat unitC: 10 × 12.011 = 120.11 g/mol. H: 8 × 1.008 = 8.06 g/mol. O: 4 × 15.999 = 64.00 g/mol.

Step 2 · Add them120.11 + 8.06 + 64.00 = 192.17 g/mol per repeat unit.

Step 3 · Divide38,400 ÷ 192.17 = 199.8, so about 200 repeat units.

Step 4 · Count the waterEach repeat unit makes two ester links, so about 400 water molecules leave while one chain is built.

Check200 × 192.17 = 38,434 g/mol, which matches the stated average closely. The monomers weigh 166.13 + 62.07 = 228.20 g/mol, and 228.20 − 2(18.02) = 192.16 g/mol. The repeat unit mass agrees.

Polymer	How it grows	Monomer	Repeat unit mass	Small molecule lost
Polyethylene	addition	ethene	28.05 g/mol	none
Polypropylene	addition	propene	42.08 g/mol	none
PVC	addition	chloroethene	62.50 g/mol	none
PTFE	addition	tetrafluoroethene	100.01 g/mol	none
Nylon-6,6	condensation	diacid + diamine	226.32 g/mol	2 H₂O
PET	condensation	diacid + diol	192.17 g/mol	2 H₂O
Protein	condensation	amino acid	57.05 g/mol for glycine	1 H₂O

Key points

Amines are weak bases, because the nitrogen lone pair can take a proton.
A ring beside the nitrogen pulls that lone pair away, so aniline is a very weak base.
Amides hold C(=O)N, are not basic, and build the peptide links in proteins.
Addition polymers come from alkenes and lose no atoms at all.
Condensation polymers need two reactive ends per monomer and lose water at each link.
Degree of polymerization = chain molar mass ÷ repeat unit molar mass.

Interactive Link monomers into addition and condensation polymers. Open full ↑

The chapter in one breath

Carbon's four bonds do the work. Chains, branches and rings all come from that one habit.
Alkanes are the baseline. C_nH_2n+2, saturated, and slow to react.
Naming has an order. Longest chain, lowest locants, branches listed alphabetically.
Isomers share a formula, not a structure. Different wiring means a different substance.
Multiple bonds mean reactivity and locked shape. That lock is where cis and trans come from.
Addition keeps every atom. Substitution makes waste. Atom economy puts a number on it.
Benzene spreads six electrons over six carbons. The 152 kJ/mol bonus is why it substitutes.
Functional groups set behavior. One O–H bond can move a boiling point by a hundred degrees.
Polymers are monomers repeated. Addition loses nothing, condensation loses water each link.

ReferenceWords worth knowing

Every term below appeared in bold somewhere above. Come back here whenever a word stops making sense.

addition polymer: A polymer built by opening double bonds and linking monomers with no atoms lost. Polyethylene is one.
alcohol: A compound with a hydroxyl group on a carbon, R–OH. Names end in -ol.
aldehyde: A carbonyl at the end of a chain, with a hydrogen on the carbonyl carbon. Names end in -al.
alkane: A hydrocarbon with single bonds only, C_nH_2n+2. Also called saturated.
alkene: A hydrocarbon with a carbon to carbon double bond, C_nH_2n. Names end in -ene.
alkyne: A hydrocarbon with a carbon to carbon triple bond, C_nH_2n-2. Names end in -yne.
amide: A nitrogen bonded straight to a carbonyl carbon. The peptide link in a protein is an amide.
amine: Ammonia with one or more hydrogens swapped for carbon groups. A weak base.
aromatic: A flat ring holding 4n + 2 delocalized electrons, which makes it unusually stable.
carbonyl group: A carbon double bonded to oxygen, C=O. The shared group of four families.
carboxylic acid: A carbonyl with an OH attached, –COOH. A weak acid with pK_a near 4 to 5.
condensation polymer: A polymer built by joining reactive ends and releasing a small molecule, usually water.
delocalization: Electrons spread over three or more atoms instead of sitting in one bond.
ester: A carboxylic acid with its OH hydrogen replaced by a carbon group, R–COO–R'.
ether: An oxygen bridging two carbon frames, R–O–R'. No O–H bond, so no donated hydrogen bonds.
functional group: The reactive piece attached to a carbon frame. It sets most of the behavior.
hydrocarbon: A compound of carbon and hydrogen only.
ketone: A carbonyl with carbon on both sides, inside the chain. Names end in -one.
monomer: The small molecule that repeats to build a polymer.
structural isomer: One of two or more compounds that share a molecular formula but connect their atoms differently.

Check yourself

1. Give the molecular formula of the straight-chain alkane with 7 carbons, and its molar mass.

C₇H₁₆, 100.20 g/mol. H = 2(7) + 2 = 16. Mass = 7(12.011) + 16(1.008) = 84.077 + 16.128 = 100.205 g/mol, so 100.20 g/mol. The name is heptane.

2. Name CH₃CH₂CH(CH₃)CH₂CH₃.

3-methylpentane. The longest chain holds five carbons, so the base is pentane. The methyl branch is on the middle carbon, which is number 3 from either end.

3. How many structural isomers does C₅H₁₂ have? Name them.

Three. Pentane is the straight chain. 2-methylbutane has a four-carbon chain with one branch. 2,2-dimethylpropane has a three-carbon chain with two branches on the middle carbon. There is no fourth way to connect five carbons.

4. Balance the combustion of propane, C₃H₈.

C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O. Carbon: 3 gives 3 CO₂. Hydrogen: 8 gives 4 H₂O. Oxygen on the right: 3(2) + 4(1) = 10 atoms, which is 5 O₂. Both sides now hold 3 C, 8 H and 10 O.

5. A hydrocarbon is C₇H₁₂. How many degrees of unsaturation does it have?

Two. A saturated C₇ alkane would hold 2(7) + 2 = 16 hydrogens. 16 − 12 = 4 missing, and 4 ÷ 2 = 2 degrees. That could be two double bonds, one triple bond, or one ring with one double bond.

6. Does propene, CH₃CH=CH₂, have cis and trans forms? Why or why not?

No. Cis and trans need two different groups on each carbon of the double bond. The CH₂ end of propene carries two identical hydrogens, so flipping it changes nothing.

7. Hydrogenating cyclohexene releases 120 kJ/mol. Benzene plus 3 H₂ releases 208 kJ/mol. Find benzene's resonance energy.

152 kJ/mol. Three separate double bonds would release 3 × 120 = 360 kJ/mol. The measured value is 208 kJ/mol. 360 − 208 = 152 kJ/mol, which is energy benzene never had to give back because delocalization had already lowered it.

8. 1-butanol boils at 117.7 °C and diethyl ether boils at 34.6 °C. Both are C₄H₁₀O. Explain the gap.

Only the alcohol has an O–H bond. 1-butanol can donate a hydrogen bond to the next molecule, so the liquid holds itself together. The ether has no O–H, so it can only accept one. Same formula, same mass, 83 degrees apart.

9. Find the pH of 0.050 M ethanoic acid. K_a = 1.8 × 10^-5.

pH = 3.02. x² ÷ 0.050 = 1.8 × 10^-5, so x² = 9.0 × 10^-7 and x = 9.49 × 10^-4 M. pH = −log(9.49 × 10^-4) = 3.02. The ionized share is 1.9%, under 5%, so the shortcut was fair.

10. Ethanoic acid reacts with 1-propanol under acid catalysis. Name both products.

Propyl ethanoate and water. The alcohol part is named first as propyl. The acid part follows as ethanoate. Losing water makes this a condensation, and adding water back would reverse it.

11. You oxidize 2-butanol. What do you get, and can you go further?

2-butanone, and no. 2-butanol is a secondary alcohol, so it oxidizes to a ketone. That carbonyl carbon has no hydrogen left to remove, so the ladder stops. A primary alcohol would have carried on to the carboxylic acid.

12. A polyethylene sample averages 42,000 g/mol. How many ethene units are in one chain?

About 1,500. The repeat unit is –CH₂CH₂–, so its mass is 2(12.011) + 4(1.008) = 24.022 + 4.032 = 28.05 g/mol. 42,000 ÷ 28.05 = 1,497, which rounds to about 1.5 × 10³ repeat units. Addition loses no atoms, so the monomer and repeat unit masses match.

Previous← Chapter 19 NextChapter 21 →