JEE Advanced 2020 · Paper 1 · Chemistry Q4 · Dronacharya Exams

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ChemistrySingle correct+3 / -1Question 22 of 54

Newman projections P, Q, R and S are shown below:

Which one of the following options represents identical molecules?

Four staggered Newman projections. P: front carbon CH₃ (up), H₃C (lower left), C₂H₅ (lower right); rear carbon HO (upper left), CH₃ (upper right), CH₃ (down). Q: front carbon C(CH₃)₂OH (up), H (lower left), C₂H₅ (lower right); rear carbon H (upper left), H (upper right), CH₃ (down). R: front carbon CH₃ (up), HO (lower left), CH₃ (lower right); rear carbon C₂H₅ (upper left), H (upper right), C₂H₅ (down). S: CH₃ (up), C₂H₅ (upper left), CH(CH₃)₂ (upper right), H (lower left), H (lower right), OH (down), i.e. CH₃ and two H on one carbon and C₂H₅, CH(CH₃)₂ and OH on the other.

A

P and Q

B

Q and S

C

Q and R

D

R and S

Answer and worked solution

Official answer: C

Read each projection as front carbon + back carbon.

P. One carbon carries $\ce{CH3, CH3, C2H5}$ , the other $\ce{OH, CH3, CH3}$ : $\ce{(CH3)2C(OH)-C(CH3)2C2H5}$ , 2,3,3-trimethylpentan-2-ol.
Q. One carbon carries $\ce{C(CH3)2OH}$ , H and $\ce{C2H5}$ ; the other is a $\ce{CH2}$ with a $\ce{CH3}$ . That is $\ce{(C2H5)2CH-C(CH3)2OH}$ , 3-ethyl-2-methylpentan-2-ol.
R. One carbon carries $\ce{OH, CH3, CH3}$ , the other $\ce{C2H5}$ , H, $\ce{C2H5}$ . Again $\ce{(CH3)2C(OH)-CH(C2H5)2}$ , 3-ethyl-2-methylpentan-2-ol.
S. One carbon is $\ce{CH2}$ with a $\ce{CH3}$ ; the other carries $\ce{C2H5}$ , $\ce{CH(CH3)2}$ and OH. That is 3-ethyl-2-methylpentan-3-ol.

None has a stereocentre, so Q and R are the same molecule. (C)

Topics:Conformations Structural isomerism

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