Organic nomenclature and functional groups: VCE Chemistry Unit 4

What this dot point is asking

VCAA wants you to draw and name members of the main organic families using IUPAC rules, to classify alcohols and amines as primary, secondary or tertiary, and to recognise structural isomers (chain, position and functional-group isomers) including drawing their structural formulas.

The answer

The main families

The functional group is the reactive part of the molecule. The rest of the molecule (the carbon skeleton) is named separately.

IUPAC naming procedure (alkane skeleton, then go up)

1. Identify the longest carbon chain that contains the principal functional group. This is the parent chain.
2. Number the chain so the principal functional group gets the lowest locant. (For multiple groups, follow the priority order: COOH > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > halide.)
3. Identify the substituents (alkyl groups, halogens, etc.) attached to the chain and their locants.
4. Combine in the form (substituents with locants alphabetical)-(parent chain)-(suffix).
5. Use commas between numbers; hyphens between numbers and letters; no spaces.

Locants are mandatory when ambiguous. Write "propan-1-ol" or "propan-2-ol", not "propanol", because the OH could be on C1 or C2.

Primary, secondary, tertiary alcohols and amines

For an alcohol: count the number of carbon atoms bonded to the C bearing the OH.

• Primary (1 deg): OH carbon bonded to 1 other C (e.g. ethanol).
• Secondary (2 deg): OH carbon bonded to 2 other C (e.g. propan-2-ol).
• Tertiary (3 deg): OH carbon bonded to 3 other C (e.g. 2-methylpropan-2-ol).

Important because only primary alcohols oxidise to aldehydes (and on to carboxylic acids), secondary alcohols oxidise to ketones (and no further), and tertiary alcohols do not oxidise under normal conditions.

For an amine: count carbons attached to the N.

• Primary: N bonded to 1 carbon (e.g. ethanamine, CH3CH2NH2).
• Secondary: N bonded to 2 carbons (e.g. N-methylmethanamine, CH3NHCH3).
• Tertiary: N bonded to 3 carbons (e.g. trimethylamine, (CH3)3N).

Structural isomers

Same molecular formula, different connectivity. Three types appear in VCE:

1. Chain isomers (skeletal): same functional group, different carbon skeleton.
   Butane (CH3CH2CH2CH3) and 2-methylpropane ((CH3)3CH) are chain isomers of C4H10.

2. Position isomers: same skeleton and same functional group, different position of the group on the chain.
   Propan-1-ol (CH3CH2CH2OH) and propan-2-ol (CH3CH(OH)CH3) are position isomers of C3H8O.

3. Functional-group isomers: same molecular formula, different functional group.
   Ethanol (CH3CH2OH) and methoxymethane (CH3OCH3, dimethyl ether) are functional-group isomers of C2H6O.

Naming esters

An ester is named in two parts: alkyl (from the alcohol) followed by alkanoate (from the carboxylic acid).

For CH3CH2COOCH3:

• The R-O- side (from methanol) gives the alkyl part: methyl.
• The R-COO- side (from propanoic acid) gives the alkanoate part: propanoate.
• Name: methyl propanoate.

Reverse the procedure to build the structure from the name. For ethyl ethanoate: ethanoic acid contributes -COO-CH2CH3 minus the OH; ethanol contributes the -OCH2CH3 part. Structure: CH3COOCH2CH3.

Examples in context

Example 1. Ester aroma volatiles in Yarra Valley pinot noir. Yarra Valley winemakers identify the ester volatiles that give pinot noir its strawberry and floral character. Ethyl ethanoate, $\text{CH}_3 \text{COOCH}_2 \text{CH}_3$, results from condensation of ethanoic acid and ethanol; aroma threshold $\sim 7 \, \text{mg/L}$, contributing nail-polish notes at higher levels. Ethyl hexanoate, $\text{CH}_3 (\text{CH}_2)_4 \text{COOCH}_2 \text{CH}_3$, brings pineapple character. The name parses as: hexanoic acid (6C, -oic acid suffix on chain 1) reacted with ethanol (2C, -ol suffix). Isoamyl acetate (3-methylbutyl ethanoate) is a chain isomer of pentyl ethanoate that smells of banana and raises wine ABV-aware tasters' eyebrows above $1 \, \text{mg/L}$. Gas chromatography confirms identity by retention time matched to authentic standards.

Example 2. Pharmaceutical isomer detection at WEHI bioavailability labs. Walter and Eliza Hall Institute drug chemists distinguish ibuprofen's R and S enantiomers because only the S-form is the active analgesic. Both share IUPAC name 2-(4-isobutylphenyl)propanoic acid: a carboxylic acid (-oic acid suffix, with the COOH carbon as C1) whose C2 carries a 4-isobutyl-substituted benzene ring. Structural and positional isomers of ibuprofen include the regioisomer 3-(4-isobutylphenyl)propanoic acid, which is biologically inactive. Functional-group isomers such as methyl 2-(4-isobutylphenyl)acetate (an ester rather than an acid) lack analgesic effect because they cannot bind the cyclooxygenase active site. The chemistry of "same formula, different structure, different function" runs through the entire pharmaceutical industry.

Try this

Q1. Name each compound: (a) $\text{CH}_3 \text{CH}_2 \text{CHO}$; (b) $\text{CH}_3 \text{CH(OH) CH}_2 \text{CH}_3$; (c) $\text{HCOOCH}_2 \text{CH}_3$. [3 marks]

• Cue. (a) Propanal. (b) Butan-2-ol (secondary). (c) Ethyl methanoate.

Q2. Draw the four structural isomers of $\text{C}_4 \text{H}_{10} \text{O}$ that are alcohols. (a) Name each. (b) Classify each as primary, secondary or tertiary. [4 marks]

• Cue. (a, b) Butan-1-ol (1°); butan-2-ol (2°); 2-methylpropan-1-ol (1°); 2-methylpropan-2-ol (3°).

Q3. Two compounds have molecular formula $\text{C}_3 \text{H}_6 \text{O}_2$. (a) Suggest a carboxylic acid and an ester of this formula. (b) Name each. (c) State one chemical test that distinguishes them. [2+2+2 marks]

• Cue. (a) $\text{CH}_3 \text{CH}_2 \text{COOH}$; $\text{HCOOCH}_2 \text{CH}_3$. (b) Propanoic acid; ethyl methanoate. (c) Add sodium carbonate: acid effervesces with $\text{CO}_2$; ester does not. Or: blue litmus turns red with acid only.