DAT Question of the Day

Correct Answer: A. Change the water solubility of an unknown compound

Adding acid or base to a mixture of unknown compounds is designed to change the water solubility. The loss or gain of protons will make the compound become ionic in nature, and thus soluble in water. On the other hand, compounds that are already ionic may become neutrally charged, resulting in lower solubility in water.

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Correct Answer: B. 15

Dynemicin A has two aromatic systems, three carbonyl substituents (two ketones, one carboxylic acid), two alkenes and two alkynes. Total = 15. Recall that alkynes possess two pi bonds in addition to one sigma bond, therefore C is not correct.

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Correct Answer: C. III

The bromonium ion is formed when alkenes react with bromine. When the π cloud of the alkene (nucleophile) approaches the bromine molecule (acting as an electrophile), the σ-bond electrons of Br2 are pushed away, resulting in the departure of the bromide anion. The species formed is a high-energy intermediate along the reaction path. It will react further with some nucleophile−methanol in this case. The electronic structure of the bromonium ion has its consequences in the stereochemistry of the reaction. Nucleophilic attack takes place from the bottom face (back-side to the “departing” bromide) on the more substituted carbon. The back-side attack leads to the anti addition (Br and Nu groups end up on the opposite faces of the starting double bond), and the nucleophile adds to the more substituted carbon (Markovnikov’s rule). Structure III corresponds to the above. Structure II may form, but only in very miniscule amounts.

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Correct Answer: B. PCC

The conversion of 2,2-dimethylbutan-1-ol to 2,2-dimethylbutanal is an oxidation (alcohol -> aldehyde). Lithium aluminum hydride (E) is a strong reducing agent. Potassium permanganate (C) is used in the oxidation of alkenes to diols, and is generally not strong enough to further oxidize aliphatic alcohols. Ozone (D) is used in the oxidative cleavage of alkenes into the corresponding aldehydes or ketones. Chromic acid (A) is a harsh oxidant, oxidizing primary alcohols up to the corresponding carboxylic acid, and secondary alcohols up to the corresponding ketone. PCC (B, pyridinium chlorochromate) is a milder chromium-based oxidant used in the selective oxidation of primary alcohols to aldehydes.

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Correct Answer: C. Hydroboration-oxidation

The electronegativity of the hydrogen atom is stronger than that of boron, which means hydrogen is relatively negatively charged compared to boron. So in this case, the hydrogen will be added to the end containing more substituents and the boron part will be added to the end containing more hydrogen atoms, which will then convert to the hydroxyl group. This is the Anti-Markovnikov product.

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Correct Answer: E. ultraviolet light

Ultraviolet–visible spectroscopy (UV-Vis or UV/Vis) refers to absorption/reflectance spectroscopy UV-Vis spectral region. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. Molecules containing π-electrons or non-bonding electrons (n-electrons) can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals. The more easily excited the electrons (i.e. lower energy gap between HOMO and the LUMO), the longer the wavelength of light it can absorb.

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Correct Answer: C. has a lower frequency than the analogous C=O in a non-conjugated system

All carbonyl compounds absorb in the IR region of 1760–1665 cm^–1 owing to the stretching vibration of the C=O bond. This carbonyl band is distinct with a characteristic high intensity, and therefore is quite useful for diagnostic purposes because few other functional groups absorb in this region. Conjugation with a double bond or benzene ring lowers the stretching frequency, usually by 30 to 40 cm^–1. The stretching frequency of the conjugated double bond is also lowered and may be enhanced in intensity. An example of this phenomenon is provided:

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Correct Answer: A. 1,1-dibromopropane

An approximated 1H NMR spectrum of 1,1-dibromopropane (CH3CH2CHBr2) is shown below:

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Correct Answer: B. The two compounds can be separated by ordinary physical chemical separation methods

Cis and trans-1,2-dibromocyclopropane (pictured below) share a diastereomeric relationship; that is, the molecules are stereoisomers that are not enantiomers. Diastereomerism occurs when two or more stereoisomers (i.e. same connectivities) of a compound have different configurations at one or more (but not all) of the equivalent stereocenters and are not mirror images of each other. Accordingly, diastereomers are distinct molecules with different chemical properties, and can be separated by ordinary physical means. Recall that enantiomers are mirror images of each other and are identical in all physical aspects except for their direction of rotation of plane polarized light.

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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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