DAT Question of the Day

Correct Answer: C. 1-bromo-3-methylbutane

A quick SN2 reminder: The SN2 reaction is a single-step displacement of a leaving group by a nucleophile. During the transition state, the bond to the nucleophile forms at the same time that the bond to the leaving group breaks. Thus, the nucleophile needs to approach from the back, and configuration at carbon is inverted. The best compound for the reaction, therefore, must be a primary alkyl halide for steric reasons. Assign the carbon centers containing the leaving group as 1°, 2°, 3° (drawing the molecules will assist you in this). C would react faster than A because of less steric hindrance around the carbon center.

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Correct Answer: C. The bromine atom in the equatorial position and the chlorine atom in the axial position.

The bromine atom is heavier and larger so it belongs in the equatorial position. Generally, the larger atoms belong in the equatorial position to minimize the steric strain. Having the bromine point away from the ring is better than having it point in an axial direction, where it will meet resistance and strain from other hydrogen atoms.

Note that the problem says this is a cis isomer, and therefore answers A, B, and E are impossible when you draw out the chair conformation. If the atoms are on the same side of the molecule, then one must be equatorial and the other axially. For stabilization purposes, it is better for the larger bromine atom to be in the equatorial position and the chlorine in the axial position.

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Correct Answer: D. inversion of configuration.

A quick SN2 reminder: The SN2 reaction is a single-step displacement of a leaving group by a nucleophile. During the transition state, the bond to the nucleophile forms at the same time that the bond to the leaving group breaks. Thus, the nucleophile needs to approach from the back, and configuration at carbon is inverted. Not only is inversion possible, it is inevitable. Inversion of the configuration always occurs in SN2 reactions. The backside attack of the nucleophile causes the inversion. Therefore if a pure enantiomer reactant is used, then the product has the opposite absolute configuration.

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Correct Answer: B. III and IV

Statements II and IV can be associated with the SN1 mechanism. Statement II is incorrect: the “1” in SN1 refers to the fact that the reaction operates under first-order kinetics (substitution, nucleophilic, unimolecular); that is, the slow (rate-determining) step is leaving group dissociation. Statement I describes the SN2 mechanism (backside attack, simultaneous nucleophile attack and leaving group dissociation).

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