MCAT Question of the Day

Correct Answer: A. 3-Ethyl-3,4-dimethylpentanoic acid

1. First, identify the (longest) parent chain -> 5 carbons; the suffix used is “pent-“.
2. Recognize that the carboxylic acid, being the only functional group, takes highest priority and is the parent functional group -> begin numbering the chain here.
3. Side chains: ethyl has locant = 3, methyl has locant = 3, 4.
4. Prefixes di-, tri- etc are NOT considered when placing substituents in alpha order.

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Correct Answer: B. 2

Carbon-centered radicals follow the same rules of stability as carbocations: the greater number of alkyl groups directly connected to a carbon atom, the greater the stabilization. Of the choices, B and D have the greater substitution around the radical carbon center (2^o). B, however, can also be described as an allylic radical; the pi system of a double bond stabilizes an adjacent radical through resonance. Remember that delocalizing the radical is a stabilizing effect.

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Correct Answer: A. LiAlH4, followed by HCl

Lithium aluminium hydride is a powerful reducing agent and can reduce all carbonyl containing groups, such as aldehydes, esters, ketones, acids, amides etc.

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Correct Answer: C. S¹⁺ and O^1-

Sulfur (S) is below oxygen in the 6th group in the periodic table. They both have 6 valence electrons and thus can fulfill the octet in the following Lewis structures:

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Correct Answer: D. All of them

The range 3500-3000 cm^-1 is typically associated with stretches between heteroatoms and a hydrogen atom, for example NH and OH bonds. They may appear broad, intense and jagged (COOH), broad and smooth (OH) or less intense and sharper (NH). The preceding sentence is likely more than you would need to know for the MCAT. In general, for MCAT purposes, it is useful to remember approx. 3300 for hydroxyl and approx. 1700 for carbonyl (C=O).

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Correct Answer: C. BF3 and Me3C⁺

A Lewis acid is an electron pair acceptor by virtue of it having an empty orbital. Both BH3 and CH3C⁺ are electron deficient, and therefore are Lewis acids. {Mnemonic: lEwis Acid = Electron Acceptor}

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Correct Answer: D. All of the above

All the mentioned forms of compounds in this question show no rotation of plane polarized light. Racemic mixtures showing no rotation of plane polarized light, like a 50-50 mix of D and L compounds, are exact racemic mixtures of two enantiomeric forms of a compound thus cancelling the polarization effect of plane polarized light allowing for optical isomerization. A meso compound is superposable on its mirror image meaning that the molecule is achiral (i.e. optically inactive) though it may contain chiral centers.

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Correct Answer: B. As pKa increases, acidity decreases.

A very low acid dissociation constant Ka means that the acid is weak and thus very little product is made. Since pKa is -logKa, the negative sign means they are always opposites. Thus low Ka means high pKa.

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Correct Answer: B. B

SN2 reactions proceed with inversion of configuration through backside displacement, so molecule A cannot be formed in this case. Molecule C is the product from elimination.

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Correct Answer: B. sp² and trigonal planar

On the surface: A carbocation is bonded to 3 substituents (i.e. ORG 4.2.1) and so it is in the middle of a flat triangle: trigonal planar.

When carbon has:
4 subtituents: sp³ and tetrahedral (109.5º)
3 subtituents: sp² and trigonal planar (120º)
2 subtituents: sp and linear (180º)

Going Deeper: There are three bonding electron pairs around the carbon atom, and these can be placed as far apart as possible in an sp² trigonal planar framework. The empty orbital, which results in the charge, is the 2pz atomic (unhybridized) orbital (ORG 1.2, 1.3; CHM 3.5).

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