Chapter 4 · Question 9

Explain the system of naming (nomenclature) of carbon compounds. Using this system, name the following compounds: (a) $\text{CH}_3 \!-\! \text{CH}_2 \!-\! \text{Br}$ , (b) $\text{CH}_3 \!-\! \text{CH}_2 \!-\! \text{COOH}$ , and (c) $\text{CH}_3 \!-\! \text{CO} \!-\! \text{CH}_3$ .

Answer Revealed

Direct Answer:

Nomenclature follows these rules: (1) Identify the number of carbon atoms in the longest chain — meth(1), eth(2), prop(3), but(4), etc. (2) Identify the functional group and use its appropriate prefix or suffix: halo (prefix chloro/bromo), alcohol (suffix -ol), aldehyde (suffix -al), ketone (suffix -one), carboxylic acid (suffix -oic acid), alkenes (suffix -ene), alkynes (suffix -yne). (3) If the suffix begins with a vowel, drop the final 'e' of the carbon chain name. (4) For unsaturation, replace 'ane' with 'ene' or 'yne'. Naming: (a)

\text{CH}_3 \!-\! \text{CH}_2 \!-\! \text{Br}

→ two carbons → ethane → prefix 'bromo' → bromoethane. (b)

\text{CH}_3 \!-\! \text{CH}_2 \!-\! \text{COOH}

→ three carbons → propane → suffix -oic acid (drop 'e') → propanoic acid. (c)

\text{CH}_3 \!-\! \text{CO} \!-\! \text{CH}_3

→ three carbons → propane → suffix -one (drop 'e') → propanone.

Simple Explanation

Naming organic compounds is like filling a form: first count the carbons (1=meth, 2=eth, 3=prop, 4=but...), then add the suffix for the functional group (alcohol → -ol, ketone → -one, acid → -oic acid). If the suffix starts with a vowel, drop the final 'e' of the chain name. So ethanol is 'eth' (2 C) + 'an' (single bonds) + 'ol' (alcohol).

\text{CH}_3\text{CH}_2\text{Br}

is bromoethane (2 C + bromo prefix).

\text{CH}_3\text{CH}_2\text{COOH}

is propanoic acid (3 C + -oic acid, drop 'e').

\text{CH}_3\text{COCH}_3

is propanone (3 C + -one, drop 'e').

Exam-Ready Structure

The IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature provides a systematic way to name every organic compound: 1. The name of a compound is based on the name of the basic carbon chain modified by a prefix or suffix indicating the nature of the functional group. 2. Steps for naming: (a) Step 1 — Identify the number of carbon atoms in the compound: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), and so on. A three-carbon chain is named 'propane' if saturated. (b) Step 2 — Indicate the functional group: Functional groups are either indicated by a prefix (e.g., chloro-, bromo- for haloalkanes) or a suffix (e.g., -ol for alcohols, -al for aldehydes, -one for ketones, -oic acid for carboxylic acids). (c) Step 3 — Vowel rule: If the suffix of the functional group begins with a vowel (a, e, i, o, u), the final 'e' of the carbon chain name is deleted and the appropriate suffix is added. Example: propane – 'e' = propan + 'one' = propanone. (d) Step 4 — Unsaturation: If the carbon chain is unsaturated (contains double or triple bonds), the final 'ane' in the name is replaced by 'ene' (for double bond) or 'yne' (for triple bond). Example: a three-carbon chain with a double bond is propene. 3. Applying the rules: (a)

\text{CH}_3\text{CH}_2\text{Br}

: two carbons → ethane. The functional group is

\!-\!\text{Br}

(bromo), used as a prefix. Name: bromoethane. (b)

\text{CH}_3\text{CH}_2\text{COOH}

: three carbons → the base is 'propane'. The functional group is

\!-\!\text{COOH}

(carboxylic acid), suffix -oic acid. Drop the final 'e' of propane → propan + oic acid = propanoic acid. (c)

\text{CH}_3\text{COCH}_3

: three carbons → propane. The functional group is

\!-\!\text{CO} \!-\!

(ketone), suffix -one. Drop the final 'e' of propane → propan + one = propanone.

Key Points

IUPAC nomenclature: name based on carbon chain modified by prefix/suffix for the functional group
Carbon count: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6)
Functional group suffixes: -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid)
Functional group prefixes: chloro-, bromo- (halogens)
Vowel rule: drop final 'e' of chain name if suffix begins with a vowel
Unsaturation: replace 'ane' with 'ene' (double bond) or 'yne' (triple bond)

Common Mistakes

Forgetting to drop the final 'e' of the alkane name when adding a suffix that begins with a vowel
Using suffix -al for ketones or -one for aldehydes — aldehyde and ketone suffixes must not be confused

Explain the system of naming (nomenclature) of carbon compounds. Using this system, name the following compounds: (a) $\text{CH}_3 \!-\! \text{CH}_2 \!-\! \text{Br}$ , (b) $\text{CH}_3 \!-\! \text{CH}_2 \!-\! \text{COOH}$ , and (c) $\text{CH}_3 \!-\! \text{CO} \!-\! \text{CH}_3$ .