Organic Chemistry — Basic Principles — Practice

Question 1

Name CH3-CH(CH3)-CH=CH2 by IUPAC rules.

Accepted Answer

The longest chain containing the double bond has 4 carbons; numbering from the C=C end gives the methyl at C-3. Name: 3-methylbut-1-ene.

Question 2

Arrange ethanol, acetic acid, phenol, and water in decreasing order of acidity.

Accepted Answer

Acetic acid (pKa ~4.8) > phenol (pKa ~10) > water (pKa ~15.7) > ethanol (pKa ~16). The carboxylate ion is most resonance-stabilised, so acetic acid is the most acidic.

Question 3

What is a chiral carbon? Give an example.

Accepted Answer

A carbon bonded to four different groups. Example: in lactic acid, CH3-C*H(OH)-COOH, the starred carbon is chiral.

Question 4

Why is a tertiary carbocation more stable than a primary carbocation?

Accepted Answer

A tertiary carbocation has three alkyl groups exerting +I effect and provides nine alpha-hydrogens for hyperconjugation, dispersing the positive charge effectively. A primary carbocation has only one alkyl group and three alpha-hydrogens, so it is far less stabilised.

Question 5

Define resonance and state the conditions for it.

Accepted Answer

Resonance is the delocalisation of pi-electrons over a conjugated system, represented by two or more contributing structures. Conditions: the molecule must be planar, have a conjugated system (alternating single and double bonds), and have available p-orbitals for overlap.

Practice — Organic Chemistry — Basic Principles

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