B. Organic Chemistry · Organic ChemistryComparing the Classes: Structure, Properties and Optical Isomers
B3.1 B3.2 B3.5 B2.1 B2.2 A1.6 A1.8About 70 minutes
Learning goals- I can identify every functional group in a molecule and name simple compounds with more than one group.
- I can compare the boiling points, solubility and odour of the organic families using intermolecular forces.
- I can identify chiral carbons and explain what optical isomers (enantiomers) are and why they matter.
- I can analyse boiling-point data to support a conclusion about intermolecular forces.
Words to know
chiral carbon (stereocentre)
A carbon atom bonded to four different atoms or groups.
chiral molecule
A molecule that cannot be superimposed on its mirror image, like a left and a right hand.
enantiomers (optical isomers)
A pair of stereoisomers that are non-superimposable mirror images of each other.
plane-polarized light
Light whose waves vibrate in only one plane; enantiomers rotate this plane by equal amounts in opposite directions.
superimposable
Able to be placed on top of another object so that every part matches exactly.
multifunctional compound
A molecule with two or more different functional groups, such as lactic acid (–OH and –COOH).
Putting the families side by side
You have now met every family in the course. Real molecules, such as medicines, flavours and the molecules of life, often contain several functional groups at once. In this lesson you will compare the families, learn to spot each group quickly, and meet the last kind of isomer: optical isomers.
| Family | Functional group | Name ending or prefix | Example | H-bonds to itself? |
|---|
| alkane | C–C only | -ane | propane | no |
| alkene | C=C | -ene | propene | no |
| alkyne | C≡C | -yne | propyne | no |
| aromatic | benzene ring | -benzene, phenyl- | methylbenzene | no |
| organic halide | C–X | fluoro-, chloro-, bromo-, iodo- | 2-chloropropane | no |
| alcohol | –OH | -ol (hydroxy-) | propan-1-ol | yes |
| ether | C–O–C | alkoxy- | methoxyethane | no |
| aldehyde | –CHO | -al | propanal | no |
| ketone | C–CO–C | -one (oxo-) | propanone | no |
| carboxylic acid | –COOH | -oic acid | propanoic acid | yes (dimers) |
| ester | –COO– | alkyl -oate | methyl ethanoate | no |
| amine | –NH₂, –NH–, –N< | -amine (amino-) | propan-1-amine | 1° and 2° only |
| amide | –CO–N< | -amide | propanamide | yes, if N–H present |
Molecules with more than one functional group
When a molecule has two or more groups, one group gets the name ending (the suffix) and the others become prefixes. A simplified priority order, highest first:
carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene/alkyne
Halogens, alkyl groups and alkoxy groups are always prefixes. The main group gets the lowest locant. Prefixes for groups that lose the contest: hydroxy- (–OH), oxo- (C=O of a ketone), amino- (–NH₂).
- CH₃CH(OH)COOH: acid beats alcohol → 2-hydroxypropanoic acid (lactic acid).
- CH₃CH(NH₂)COOH: acid beats amine → 2-aminopropanoic acid (the amino acid alanine).
- CH₂=CHCH₂OH: alcohol beats alkene → prop-2-en-1-ol (the C=C locant goes before -en-, the –OH locant before -ol).
Spotting groups is a skill you will use constantly in biology, medicine and pharmacy. Take it slowly: look for each heteroatom (O, N, halogen) and check what it is bonded to.
Worked exampleExample 1: Identifying functional groups
Identify the functional groups in (a) vanillin, which has a benzene ring with –CHO, –OH and –OCH₃ attached; (b) acetaminophen, which has a benzene ring with –OH and –NH–CO–CH₃ attached.
- (a) –CHO: aldehyde. –OH on a benzene ring: a phenol group (an –OH like an alcohol's, but on an aromatic ring). –OCH₃: an ether (O between the ring carbon and CH₃).
- (b) –OH on the ring: phenol. –NH–CO–CH₃: the N is bonded to a C=O, so it is an amide (not an amine).
- Both also contain an aromatic ring.
(a) aldehyde, ether, phenol (aromatic). (b) amide, phenol (aromatic).
Comparing physical properties
All the patterns in this unit come from intermolecular forces:
- London forces act between all molecules and grow with size.
- Dipole-dipole forces act between polar molecules (C–X, C–O, and especially C=O).
- Hydrogen bonds form only when H is bonded to O or N (or F). Molecules that can both donate and accept hydrogen bonds stick together most strongly.
For molecules of similar size, boiling points generally rise in this order:
hydrocarbons < ethers and halides < aldehydes, ketones, esters and amines < alcohols < carboxylic acids and amides
The middle group overlaps: the weak N–H···N hydrogen bonds of amines have about the same effect as the strong C=O dipoles of aldehydes and ketones.
Solubility in water depends on whether a molecule can hydrogen-bond with water (as donor or acceptor). That is why ethers, aldehydes, ketones and esters, which cannot hydrogen-bond to themselves, can still dissolve in water when they are small. In every family, solubility drops as the non-polar carbon chain grows.
Odour: a substance must be volatile to be smelled. Low-boiling compounds such as esters, aldehydes, ketones and amines often have strong odours; carboxylic acids smell sharp; large molecules and polymers have little or no smell.
| Compound | Family | Molar mass (g/mol) | Boiling point (°C) |
|---|
| butane | alkane | 58 | −1 |
| methoxyethane | ether | 60 | 7 |
| methyl methanoate | ester | 60 | 32 |
| propanal | aldehyde | 58 | 48 |
| propan-1-amine | amine (1°) | 59 | 48 |
| propanone | ketone | 58 | 56 |
| propan-1-ol | alcohol | 60 | 97 |
| ethanoic acid | carboxylic acid | 60 | 118 |
| ethanamide | amide (1°) | 59 | 221 |
Worked exampleExample 2: Analysing data (A1.6, A1.8)
Use the data table to answer: (a) Which variable has been controlled, and why? (b) A student claims, "Amines always boil higher than ketones, because amines can hydrogen-bond." Does the data support this? (c) Explain the amide's very high value.
- (a) Molar mass (about 58 to 60 g/mol), so London forces are similar and the effect of the functional group can be seen.
- (b) No. Propan-1-amine (48 °C) boils below propanone (56 °C) and at the same temperature as propanal. N–H···N hydrogen bonds are weak (N is less electronegative than O), and the C=O dipole of a ketone is strong, so their effects are similar. One data point can disprove an "always" claim.
- (c) Ethanamide has two N–H donors and a strongly polar C=O acceptor, so each molecule forms several strong hydrogen bonds; it is even a solid at room temperature.
Molar mass is controlled; the claim is not supported (amines and ketones overlap); amides are highest because of extensive hydrogen bonding.
Optical isomers: molecules with handedness
Hold your hands up, palms facing you. They are mirror images, but you cannot stack your left hand on your right so that every part matches: a left glove does not fit a right hand. Objects like this are chiral (from the Greek for "hand").
A carbon atom bonded to four different groups is a chiral carbon (or stereocentre). A molecule with one chiral carbon exists as two non-superimposable mirror images, called enantiomers or optical isomers.
- Butan-2-ol, CH₃CH(OH)CH₂CH₃: C2 is bonded to H, OH, CH₃ and CH₂CH₃, four different groups → chiral.
- Propan-2-ol, CH₃CH(OH)CH₃: C2 has two identical CH₃ groups → not chiral.
Enantiomers have the same boiling point, melting point, density and solubility. They differ in two ways:
- They rotate plane-polarized light by equal amounts in opposite directions (hence "optical" isomers).
- They interact differently with other chiral molecules, including the receptors in your nose and the enzymes in your body. One enantiomer of carvone smells of spearmint; the other smells of caraway seeds. In the pain reliever ibuprofen, the S enantiomer does most of the pain-relieving work. This is why drug companies must study each enantiomer of a new medicine separately.
In university you will name enantiomers with the R/S system.
Worked exampleExample 3: Finding chiral carbons
Which of these have a chiral carbon? (a) 2-chlorobutane, (b) 2-chloropropane, (c) 2-hydroxypropanoic acid (lactic acid), (d) aminoethanoic acid (glycine, H₂NCH₂COOH).
- (a) CH₃CHClCH₂CH₃: C2 has H, Cl, CH₃, CH₂CH₃ → chiral.
- (b) CH₃CHClCH₃: C2 has two CH₃ → not chiral.
- (c) CH₃CH(OH)COOH: C2 has H, OH, CH₃, COOH → chiral. (Your muscles make one enantiomer.)
- (d) H₂N–CH₂–COOH: the CH₂ has two H → not chiral. Glycine is the only common amino acid that is not chiral.
(a) and (c) have chiral carbons; (b) and (d) do not.
Hands-on activityLeft-handed molecules
You need: 10 gumdrops or clay balls in 5 colours (2 of each colour), 8 toothpicks, a mirror. Grown-up note: toothpicks are sharp.
- Choose one colour as "carbon". Build a tetrahedral carbon with four different colours on its four toothpicks.
- Hold it in front of the mirror. Build a second model that looks exactly like the reflection.
- Try to rotate the second model so that all four colours line up with the first one. Can you?
- Now swap any two colours on the second model. Can you make it match now?
- Rebuild one model with two balls of the same colour. Build its mirror image. Can you superimpose these?
What to look for: with four different groups, the two models cannot be superimposed: they are enantiomers. Swapping any two groups turns one enantiomer into the other. With two identical groups, the mirror image can be rotated to match: the molecule is not chiral.
Just for funThe two enantiomers of carvone have the same formula, the same boiling point and the same density, yet one smells like spearmint gum and the other like rye bread with caraway seeds. Your nose can tell left from right better than many lab instruments.
Your turn
1Identify the functional groups in HOCH₂CH₂COOH and name it.
alcohol (–OH) and carboxylic acid (–COOH). The acid is the main group: 3-hydroxypropanoic acid.
2Rank by boiling point: propanal, propan-1-ol, butane. Explain.
butane < propanal < propan-1-ol: London only; dipole-dipole (C=O); hydrogen bonds (O–H).
3Which can hydrogen-bond with water but not with itself: ethyl ethanoate or ethanol?
ethyl ethanoate (its O atoms accept H-bonds from water; it has no O–H). Ethanol does both.
4Does pentan-3-ol have a chiral carbon? Explain.
CH₃CH₂CH(OH)CH₂CH₃: C3 has H, OH and two identical ethyl groups → no.
5Does pentan-2-ol have a chiral carbon?
C2 has H, OH, CH₃ and CH₂CH₂CH₃, four different groups → yes.
Ketone beats alcohol. 4-carbon chain, C=O on C2, OH on C4: 4-hydroxybutan-2-one.
7How do two enantiomers differ in their physical properties?
They have the same mp, bp, density and solubility; they rotate plane-polarized light in opposite directions and can interact differently with other chiral molecules.
More practice: warm-up
1Which family has the –COO– group?
2Which prefix names an –OH group that is not the main group?
3What makes a carbon atom chiral?
It is bonded to four different groups.
4Which force acts between all molecules?
London dispersion forces.
5Which families can hydrogen-bond to themselves?
alcohols, carboxylic acids, 1° and 2° amines, and amides with N–H.
More practice: on your own
Acid is the main group; –NH₂ on C3: 3-aminopropanoic acid (β-alanine).
2Identify all functional groups in CH₂=CHCOOCH₃ and name it.
alkene (C=C) and ester (–COO–). Acid part: propenoate (3 C with C=C); alcohol part: methyl. methyl propenoate (methyl acrylate).
3Explain why propanone (56 °C) boils higher than methoxyethane (7 °C), although neither can hydrogen-bond to itself.
Propanone's C=O bond is much more polar than the C–O bonds of the ether, so propanone has much stronger dipole-dipole forces.
4How many chiral carbons does 2,3-dichlorobutane, CH₃CHClCHClCH₃, have?
C2: H, Cl, CH₃, CHClCH₃ (four different); C3: H, Cl, CH₃, CHClCH₃ (four different). Two chiral carbons.
5A student says, "Any compound with an –OH group is an alcohol." Use examples to evaluate this.
Not true. In a carboxylic acid the –OH is part of –COOH (bonded to a C=O carbon); in phenol the –OH is on a benzene ring and has different properties. An alcohol's –OH must be on a saturated (sp³) carbon.
Challenge
1A student mixed equal amounts of two enantiomers of a drug and found the mixture did not rotate plane-polarized light. Explain, and explain why the mixture might still have different biological effects from the pure active enantiomer.
The two enantiomers rotate the light by equal amounts in opposite directions, so a 50:50 mixture (a racemic mixture) shows no net rotation. In the body, the mixture contains only half as much of the active enantiomer, and the other enantiomer may be inactive, or may cause different effects, because receptors and enzymes are themselves chiral.
2Propose structures for a C₃H₆O₂ compound that is (a) acidic, (b) a sweet-smelling, low-boiling liquid that does not react with NaHCO₃, (c) a compound with both an alcohol and an aldehyde group.
(a) propanoic acid, CH₃CH₂COOH. (b) an ester: methyl ethanoate, CH₃COOCH₃, or ethyl methanoate, HCOOCH₂CH₃. (c) 3-hydroxypropanal, HOCH₂CH₂CHO (or 2-hydroxypropanal, CH₃CH(OH)CHO, which is chiral).
3Use intermolecular forces to explain why methyl methanoate (60 g/mol) boils at 32 °C, far below its isomer ethanoic acid (118 °C), but well above butane (58 g/mol, −1 °C).
Both isomers are polar, but only ethanoic acid has an O–H, so it forms strong hydrogen-bonded dimers; the ester cannot hydrogen-bond to itself. Compared with butane, the ester has polar C=O and C–O bonds and so adds dipole-dipole forces to similar London forces, raising its boiling point above butane's.
Quiz
1. In 2-hydroxypropanoic acid, which group determines the name ending?
2. Which compound has a chiral carbon?
3. Which list is in order of increasing boiling point for molecules of similar mass?
4. What property differs between two enantiomers?
5. Acetaminophen has –NH–CO–CH₃ on a ring. What group is this?
6. Which compound can accept hydrogen bonds from water but cannot hydrogen-bond to itself?
Big ideas (B3.1, B3.2, B3.5): the families differ in their functional groups, and those groups control the intermolecular forces and so the properties. Molecules with a carbon bonded to four different groups are chiral and exist as enantiomers, which matter greatly in biology and medicine.
How to help: Pick a product label (a medicine or flavour), look up a structure together, and ask your teen to name every functional group. Use your hands to discuss chirality.
Common difficulties:
- Calling an amide an amine, or a carboxylic acid an alcohol, because of an N or OH.
- Assuming hydrogen bonding wherever there is an H and an O (the H must be on the O or N).
- Missing that two identical groups (such as two CH₃) make a carbon non-chiral.
- Treating trends as laws: real data have exceptions, and good scientists say so (A1.8).