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Which of the following is not chiral :-
A
2-Butanol
B
2,3-Dibromopentane
C
3-Bromopentane
D
2-Hydroxypropanoic acid
Detailed Solution
A molecule is chiral if it has a carbon atom attached to four different groups (and no plane of symmetry).
3-Bromopentane: $CH_3CH_2-CH(Br)-CH_2CH_3$. Carbon 3 carries $H$, $Br$ and two identical ethyl groups, so it is not a chiral carbon, and the molecule has a plane of symmetry.
So 3-bromopentane is not chiral.
2-Butanol: $CH_3-\overset{*}{C}H(OH)-CH_2CH_3$. Carbon 2 has $H$, $OH$, $CH_3$ and $C_2H_5$, so it is chiral.
2,3-Dibromopentane: $CH_3-\overset{*}{C}HBr-\overset{*}{C}HBr-CH_2CH_3$ has two different chiral carbons and no plane of symmetry, so it is chiral.
2-Hydroxypropanoic acid (lactic acid): $CH_3-\overset{*}{C}H(OH)-COOH$ has $H$, $OH$, $CH_3$ and $COOH$ on carbon 2, so it is chiral.
Hence the compound that is not chiral is 3-bromopentane.
3-Bromopentane: $CH_3CH_2-CH(Br)-CH_2CH_3$. Carbon 3 carries $H$, $Br$ and two identical ethyl groups, so it is not a chiral carbon, and the molecule has a plane of symmetry.
So 3-bromopentane is not chiral.
2-Butanol: $CH_3-\overset{*}{C}H(OH)-CH_2CH_3$. Carbon 2 has $H$, $OH$, $CH_3$ and $C_2H_5$, so it is chiral.
2,3-Dibromopentane: $CH_3-\overset{*}{C}HBr-\overset{*}{C}HBr-CH_2CH_3$ has two different chiral carbons and no plane of symmetry, so it is chiral.
2-Hydroxypropanoic acid (lactic acid): $CH_3-\overset{*}{C}H(OH)-COOH$ has $H$, $OH$, $CH_3$ and $COOH$ on carbon 2, so it is chiral.
Hence the compound that is not chiral is 3-bromopentane.