DAT Question of the Day

Correct Answer: A. I

Of the possibilities, all have the molecular formula C8H12O, but only compounds I and III can have 5 chemically unique proton signals in the H NMR spectrum. The broad IR peak at 3400 (indicative of alcohol), and sharp peak at 3300 (alkyne C-H stretch) confirms the compounds identity as I.

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Correct Answer: C. The polarity of a molecule is dependent on its 3-D structure

A molecule’s dipole is an electric dipole with an inherent electric field (not be confused with a magnetic dipole which generates a magnetic field). Molecules can have dipole moments due to non-uniform distributions of positive and negative charges on the various atoms. For diatomic molecules there is only one (single or multiple) bond so the bond dipole moment is the molecular dipole moment, with typical values in the range of 0 to 11 D. For polyatomic molecules there is more than one bond, and the total molecular dipole moment may be approximated as the vector sum of all individual bond dipole moments.

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Correct Answer: C. 3 + 6 only

The cyclohexene product is indicative of a Diels-Alder reaction, the cycloaddition of a conjugated diene (electron rich) and dienophile (electron poor) components. For best reactivity, the diene must be able to adopt a cisoid conformation. The reaction is stereospecific with respect to both diene and dienophile, meaning a cis-dienophile gives cis substitution in the product (same applies for trans-dienophiles -> trans product substitution). If diene substituents have the same stereochemistry (i.e. both alkenes are E), both substituents will end up on the same face of the product. Mixed stereochemistry (i.e. E and Z) in the diene results in substituents on opposite faces of the product. The final product has syn-substituted diene components (diene is either E,E or Z,Z), and dienophile substituents are trans, (original dienophile has trans geometry). Hence, only answer choice [C] contains reactants that will form the desired product, with reactant 3 as the E,E diene, and reactant 6 as the trans dienophile. Answer choice [B] states reactants 1 and 6 will produce the final product in high yield, however, reactant 1 contains mixed stereochemistry (E,Z bonds) and will result in the methyl substituents on opposite faces of the product.

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Correct Answer: A. 1

Simple hydrocarbons (containing only C and H) are weakly acidic in comparison to other organic compounds. However, carbanions can be formed, with their stability depending quite heavily on the hybridization state of the orbital containing the lone pair of electrons. Alkyne 1 is most acidic as the lone pair of the conjugate base is held in a sp-orbital, rather than sp3 (for 2 and 5) or sp2 orbital (for 3 and 4). Having a greater proportion of “s character” allows for the electron density in an sp-orbital to be held closer to the positively charged nucleus, making it electrostatically more stable.

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Correct Answer: E. Addition of a nucleophile to the carbonyl is followed by loss of the leaving group.

Carbonyl compounds react with nucleophiles via an addition mechanism: the nucleophile attacks the carbonyl carbon, forming a tetrahedral intermediate. The tetrahedral intermediate itself can be an alcohol or alkoxide, depending on the pH of the reaction. The tetrahedral intermediate of an acyl compound contains a substituent attached to the central carbon that can act as a leaving group. After the tetrahedral intermediate forms, it collapses, recreating the carbonyl C=O bond and ejecting the leaving group in an elimination reaction. In nucleophilic acyl substitution, the leaving group is ejected in a separate exothermic step. The Hammond Postulate predicts that the transition state for this step resembles the tetrahedral intermediate. Therefore, in the transition state for this step minimal bond breaking has occurred, and the importance of the nature of the leaving group is diminished.

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Correct Answer: A. acetylene

Bond length is the distance between the centers of two covalently bonded atoms and is mostly dependent on atomic size, bond order (length: single > double > triple), and hybridization (C–H and C–C bonds shorten slightly with increased s character on carbon).

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Correct Answer: B. 10

Recall that the degrees of unsaturation corresponds to the sum of pi bonds and/or rings present in a molecule. Each ring and each pi bond contributes one degree of unsaturation; triple bonds, possessing one sigma and two pi bonds, contribute two degrees of unsaturation. Gestrinone has four rings (4), three alkenes (3), one carbonyl (1) and one alkyne (2). Thus IHD = 4 + 3 + 1 + 2 = 10

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Correct Answer: E. 1,4-dimethylcyclopent-1-ene

The treatment of alkenes with acidic potassium permanganate under high temperatures results in oxidative cleavage. The nature of the products are dependent on alkene substitution, but all with possess a carbonyl functionality (carboxylic acid, ketone, CO2). Looking at the product, one can identify the two carbon atoms of the original alkenyl compound, and work back accordingly. Finally, standard IUPAC naming will apply.

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Correct Answer: D. I and III

Aromatic compounds follow four rules: (1) They are conjugated—there needs to one “p” orbital from each atom in the ring, so each atom must be either sp² or sp hybridized; 2) They are cyclic: linear systems are not aromatic; 3) They are planar: there is good overlap/interaction between p orbitals; 4) they follow the The Huckel Rule—4n+2 π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc.). Of the possibilities,compounds I (14 e⁻, n = 3) and III (10 e⁻, n = 2) obey the rule.

This is a little advanced and at the top of the difficulty the DAT can throw at you, you have to consider compound 3 as one molecule, not two separate rings. Aromaticity requires a planar conjugated system, so we can ignore the bridge carbon sticking out of the plane of the rest of the molecule. There cannot be any sp3 atoms IN the aromatic ring. The carbon bridge actually pushes the 10 carbon ring to be planar, without it the molecule wouldn’t be aromatic.

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Correct Answer: D. The polarity of a molecule is dependent on its 3-D structure

A molecule’s dipole is an electric dipole with an inherent electric field (not be confused with a magnetic dipole which generates a magnetic field). Molecules can have dipole moments due to non-uniform distributions of positive and negative charges on the various atoms. For diatomic molecules there is only one (single or multiple) bond so the bond dipole moment is the molecular dipole moment, with typical values in the range of 0 to 11 D. For polyatomic molecules there is more than one bond, and the total molecular dipole moment may be approximated as the vector sum of all individual bond dipole moments.

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