IB DP Chemistry 20.3 Stereoisomerism HL Paper 1

Question

Which compound can exist as stereoisomers?

A.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CHO}}\)

B.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COC}}{{\text{H}}_{\text{3}}}\)

C.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CH(C}}{{\text{H}}_{\text{3}}}{{\text{)}}_{\text{2}}}\)

D.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CHOHC}}{{\text{H}}_{\text{3}}}\)

▶️Answer/Explanation

D

Butan-2-ol, also known as sec-butanol or 2-butanol, does exhibit stereoisomerism. It has a chiral center at the second carbon atom (counting from the left or right, excluding the hydroxyl group). 

The carbon atom labeled with an asterisk (*) represents the chiral center. Since this carbon atom is bonded to four different groups (H, CH3, CH2OH, and another carbon), it can exist in two different stereoisomeric forms. 

The two possible stereoisomers of butan-2-ol are known as (R)-2-butanol and (S)-2-butanol.

Question

Which statement is correct about the enantiomers of a chiral compound?

A.     Their physical properties are different.

B.     All their chemical reactions are identical.

C.     A racemic mixture will rotate the plane of polarized light.

D.     They will rotate the plane of polarized light in opposite directions.

▶️Answer/Explanation

D

The enantiomers of a chiral compound rotate the plane of polarized light in opposite directions. Enantiomers have the same physical properties. However, diastereomers which are stereoisomers that are not enantiomers can have different physical properties.

The chemical reactions of enantiomers do not necessarily have to be the same. While enantiomers have the same physical and chemical properties, except for their interaction with plane-polarized light, their reactivity in chemical reactions can differ.

A racemic mixture, which is a 50:50 mixture of enantiomers, will not rotate the plane of polarized light. Enantiomers are mirror images of each other and have the same physical and chemical properties, except for their interaction with plane-polarized light. When a pure enantiomer is exposed to plane-polarized light, it will rotate the plane of polarization in a specific direction, either clockwise (dextrorotatory, designated as “+”) or counterclockwise (levorotatory, designated as “-“). When equal amounts of both enantiomers are combined to form a racemic mixture, the rotations caused by the individual enantiomers cancel each other out. The net result is that a racemic mixture does not exhibit any optical activity and does not rotate the plane of polarized light. The specific rotation of a racemic mixture is zero.

Question

Which compound could rotate the plane of polarization of polarized light?

A.     \({{\text{(C}}{{\text{H}}_{\text{3}}}{\text{)}}_{\text{2}}}{\text{CHC}}{{\text{H}}_{\text{2}}}{\text{Cl}}\)

B.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{Cl}}\)

C.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{CHClC}}{{\text{H}}_{\text{3}}}\)

D.     \({{\text{(C}}{{\text{H}}_{\text{3}}}{\text{)}}_{\text{3}}}{\text{CCl}}\)

▶️Answer/Explanation

C

2-chlorobutane (CH3CH2CHClCH3) can exhibit optical isomerism. 2-chlorobutane does possess an asymmetric carbon atom (chiral center) and can exist as optical isomers. The carbon atom bonded to the chlorine atom is the chiral center in this molecule. 

The carbon atom marked with an asterisk (*) is the chiral center. Since it is bonded to four different groups (H, CH3, CH2CH3, and Cl), it can exist as two different stereoisomers. These stereoisomers of 2-chlorobutane are known as (R)-2-chlorobutane and (S)-2-chlorobutane.

Question

Which statement about stereoisomers is correct?

A. 1,2-dichloroethane has two geometrical isomers.

B. 1,2-dichloroethane has two optical isomers.

C. 1,2-dichloroethene has two geometrical isomers.

D. 1,2-dichloroethene has two optical isomers.

▶️Answer/Explanation

C

1,2-dichloroethane (CH2Cl-CH2Cl) can exist as two stereoisomers: the cis isomer and the trans isomer. These isomers differ in the spatial arrangement of the chlorine atoms around the central carbon-carbon bond.

1,2-dichloroethene does not exhibit stereoisomerism as it lacks a chiral center. It is an example of a molecule that possesses geometric isomerism. 1,2-dichloroethene, also known as cis-1,2-dichloroethene or trans-1,2-dichloroethene, refers to the two geometric isomers that differ in the arrangement of the chlorine atoms around the double bond.

Question

Which molecule has a chiral center?

A.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CH=CHCHO}}\)

B.     \({{\text{(C}}{{\text{H}}_{\text{3}}}{\text{)}}_{\text{2}}}{\text{C=CHC}}{{\text{H}}_{\text{2}}}{\text{OH}}\)

C.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{OC}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}\)

D.     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CHOHC}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{3}}}\)

▶️Answer/Explanation

D

Butan-2-ol, also known as sec-butanol or 2-butanol, does exhibit stereoisomerism. It has a chiral center at the second carbon atom (counting from the left or right, excluding the hydroxyl group). 

The carbon atom labeled with an asterisk (*) represents the chiral center. Since this carbon atom is bonded to four different groups (H, CH3, CH2OH, and another carbon), it can exist in two different stereoisomeric forms. 

The two possible stereoisomers of butan-2-ol are known as (R)-2-butanol and (S)-2-butanol.

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