Organic Chem (Isomerism) Quiz

How many σ (sigma) bonds are present in one molecule of methylpropene?

• 11

• 12

• 6

• 4

Answer explanation

Draw out the displayed formula of methylpropene.

Note that each -CH₃ substituent has 3 σ bonds too.

What is the value of the bond angle, x in methylpropene?

• 120^o

• 180^o

• 109^o

• 90^o

Answer explanation

Geometry of sp² hybridised C atom (of C=C bond) is trigonal planar hence the value of x is 120^o.

What is the IUPAC name of the following compound?

• 2-methylhept-5-ene

• 5-ethylhex-2-ene

• 5-ethylhex-3-ene

• 5-methylhept-2-ene

Answer explanation

The longest continuous carbon chain consists of 7 C atoms hence the parent chain is hept-2-ene.

What are the numbers of sp³ and sp² hybridised C atoms in Compound J?

• 12 sp³ and 0 sp² hybridised C

• 10 sp³ hybridised and 2 sp² hybridised C

• 9 sp³ hybridised and 3 sp² hybridised C

• 8 sp³ hybridised and 4 sp² hybridised C

Answer explanation

Other than these three sp² hybridised C atoms, the remaining C atoms in Compound J are all sp³ hybridised.

How many constitutional isomers are possible for a compound of molecular formula C₄H₉Br?

• 2

• 3

• 4

• 5

Answer explanation

2-bromobutane can exist as a pair of enantiomers. (C2 is chiral)

There are a total of 5 isomers (both constitutional and stereoisomers) with molecular formula C₄H₉Br.

But since the question only asks for constitutional isomers, the answer is 4.

Predict whether cinnamaldehyde can exhibit cis-trans isomerism. Briefly explain your answer.

Answer explanation

How many chiral centres in the isomer of butane-1,2-diol shown below?

• 0

• 1

• 2

• 4

Why is this particular isomer, named meso-butane-1,2-diol not able display optical activity?

• There is no chiral carbon in this isomer.

• There is an internal plane of symmetry in the isomer.

• This isomer has a non-superimposable mirror image.

• This isomer is not able to rotate plane-polarised light.

Answer explanation

When 1-methylcyclohexene is added to bromine in an inert organic solvent, there is a fast reaction that decolourises the bromine. The product of this reaction, 1,2-dibromo-1-methylcyclohexane, exists as a mixture of stereoisomers. .

How many chiral centres are present in 1,2-dibromo-1-methycyclohexane? Mark the chiral centres with *.

What is the maximum number of stereoisomers of 1,2-dibromo-1-methylcyclohexane?

• 2

• 4

• 8

• 16

Answer explanation