DAT Organic Chemistry (Natural Sciences) Practice Answers

<p><strong>Answers and explanations</strong><br />
<strong>1. D:</strong> Pure compounds are characterized by having sharp melting points. Eutectic mixtures are also characterized by a distinct melting point associated with a specific combination of two or more different compounds. Thus a sharp melting point is not sufficient by itself to identify an unknown substance as a pure compound. Only b. and c. can be unequivocally true statements</p>
<p><strong>2. D:</strong> The formula contains six carbon atoms and six hydrogen atoms. Structure a. corresponds to the formula C<span class=”style1″>6</span>H<span class=”style1″>12</span>. Structures b. and c. correspond to C<span class=”style1″>6</span>H<span class=”style1″>8</span>. Only structure d. of these possibilities corresponds to the formula C<span class=”style1″>6</span>H<span class=”style1″>6</span>.</p>

<p><strong>3. B: </strong>In this reaction, the electron-rich C=C bond of 1-methylcyclohexene accepts a proton from the strong acid solution forming a C-H bond between one of the C=C carbon atoms and the H+ ion. This results in the formation of a 2<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> carbonium ion at C2 if the C-H bond forms on C1 or a 3<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> carbonium ion at C1 if the C-H bond forms on C2. The process is a reversible equilibrium, and since a 3<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> carbonium ion is more stable than a 2<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> carbonium ion, any 2<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> ions formed would rearrange to the 3<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> ion structure. Addition of H<span class=”style4″>2</span>O to the 3<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ /> carbonium ion, followed by loss of H+, produces the product 1-methylcyclohexanol.</p>
<p><strong>4. A:</strong> This is the definition of the carbonyl functional group. The term “carbonyl” refers specifically to the C=O group. The other three each contain a carbonyl group, but are not of themselves a carbonyl group. The carbonate group is the CO32- ion, and the term “carbonate” is used to refer to salts and esters of the corresponding carbonic acid. The carboxylic function is the –COOH group. The acyl group is the carboxylic function without the -OH and can be represented by RCO. It is a carbonyl bonded to a carbon group (R). Ketones, aldehydes, esters and amides all contain acyl groups.</p>
<p><strong>5. B:</strong> The Z- designation stands for “zusammen” (German, “the same”) and indicates that the two high priority substituents are located on the same side of the molecular structure relative to each other. The E- designation stands for “entgegen” (German, “opposite”) and indicates that the high priority substituents are located on opposite sides of the molecular structure relative to each other. The R- and d,l designations indicate the absolute configuration and the direction of optical rotation about an asymmetric <img src=”images/sp3.jpg” alt=”sp3″ width=”20″ height=”13″ /> carbon atom.</p>
<p><strong>6. B:</strong> An asymmetrically substituted sp3 carbon atom is optically active and rotates the plane of plane-polarized light either clockwise or counter-clockwise. The order of priority of substituents about the axis of one of the bonds is either clockwise or counter-clockwise. The atom is thus described as having “handedness” or “chirality”, as in left-handed or right-handed. It is a chiral center. A conformer is a conformational isomer, such as the chair and boat conformations of the cyclohexane ring. It is a reactive center only if it is the location in the molecule at which a particular reaction occurs, which is not always the case. Stereomer, or stereoisomer, describes the relative configurations of two or more chiral centers in a molecule.</p>
<p><strong>7. A:</strong> The main bond between two <em>sp</em> or <img src=”images/sp2.jpg” alt=”sp2″ width=”21″ height=”14″ /> hybridized atoms is the <img src=”images/sigma.jpg” alt=”sigma” width=”13″ height=”10″ /> (sigma. bond directly between them. A second bond is formed between the two atoms by the sideways overlap of the adjacent p orbitals. This is called a <img src=”images/pi.jpg” alt=”pi” width=”10″ height=”9″ /> (pi) bond. Electrons in the adjacent <em>p</em> orbitals are able to move freely, or delocalize, through the <img src=”images/pi.jpg” alt=”pi” width=”10″ height=”9″ /> bond system.</p>
<p><strong>8. C:</strong> Nitriles are characterized by the –CN functional group, in which the carbon atom is triply bonded to the nitrogen atom. Alkynes are characterized by a triple bond between two carbon atoms. Cycloalkenes are ringed structures which have a double bond between two carbon atoms. Alkenones contain both carbon-carbon double bonds and ketone carbonyl groups. “Aromatic” refers to the electronic structure of certain molecules, such as benzene, and describes the greater stability of the molecule due to the delocalization of electrons between adjacent and overlapping p-orbitals. </p>
<p><strong>9. B: </strong>The oxidizing agent in Tollen’s reagent is silver ion from silver oxide, A<span class=”style3″>g<span class=”style4″>2</span></span>O. It will oxidize an aldehyde to a carboxylic acid. In this redox reaction, the silver ion is reduced from Ag+ to Ag<img src=”images/0.jpg” alt=”0″ width=”5″ height=”10″ />, or neutral silver atoms. This is a classic characterization test in which a positive result is indicated by the formation of a silver ‘mirror’ coating in the test tube.</p>
<p><strong>10. B: </strong>The classic method of making ‘soap’ is to treat animal fats and vegetable oils with a water solution of ‘lye’, aqueous sodium hydroxide. The fats and oils are composed of triglycerides, triesters of glycerol and long-chain carboxylic acids. Saponification, which literally means ‘soap-making’, is the base-catalyzed hydrolysis of ester linkages.<br />
Elimination reactions eject the components of a small molecule from a larger one, as for example the elimination of HBr from a bromoalkane to form an alkene.<br />
Inversion refers to the reversal of the order of substituents about a carbon atom in a substitution reaction. Dehydrogenation is the elimination of H<span class=”style1″>2</span> from a molecule.</p>
<p><strong>11. B:</strong> In an S<span class=”style4″>N2</span> reaction, a nucleophile approaches an <img src=”images/sp3.jpg” alt=”sp3″ width=”20″ height=”13″ /> carbon center from the side opposite the substituent that it will replace. As it approaches, the bond to that “leaving group” weakens and a bond to the approaching nucleophile begins to form. As this change progresses, the hybridization of the atomic orbitals on the reactive center changes from <img src=”images/sp3.jpg” alt=”sp3″ width=”20″ height=”13″ /> to <img src=”images/sp2.jpg” alt=”sp2″ width=”21″ height=”14″ /> <em>+ p</em>, and the other three substituent’s move into a trigonal planar array about the carbon atom in the transition state. As the bond continues to form to the nucleophile, the bond to the leaving group disappears and the hybridization of the central carbon atom becomes <img src=”images/sp3.jpg” alt=”sp3″ width=”20″ height=”13″ /> again. The three substituent’s that were initially oriented toward the incoming nucleophile end up oriented away from it in the product structure. This is inversion of the stereochemistry. The formation of a carbonium ion is characteristic of S<span class=”style4″>N1</span> and E1 reaction mechanisms. Alcohols can be produced by an S<span class=”style4″>N2</span> reaction, but so can many other types of compounds. Carbon-carbon double bonds are formed in elimination reactions, not substitution reactions.</p>
<p><strong>12. B:</strong> Separation of compounds in distillation occurs because of their differential rates of evaporation and condensation equilibria. The more often the vapors in the apparatus can condense and evaporate, the more distinct the separation of the components will be.<br />
The more quickly a distillation process is carried out, the less distinct the separation of the components will be, making the process less efficient. A very cold condenser temperature has the effect of forcing the vapor through the system before it can condense and evaporate again, decreasing both the separation of the components and the efficiency of the process. Reduced pressure decreases the temperatures at which the components will distill, without affecting the separation behavior.</p>
<p><strong>13. C:</strong> Monosaccharides are simple sugar molecules, or single carbohydrate molecules, such as glucose and fructose. They are cyclic compounds but do not have a carbonyl group when they are in that form, so they cannot be classified as either ketones or aldehydes. They have several –OH groups, as carbohydrates do, but they are not carboxylic acids.</p>
<p><strong>14. B: </strong>Furan is a five-membered oxygen heterocyclic compound. Sugar molecules that have this same basic skeletal structure are called furanose sugars. Pyran is a six-membered ring analog of furan. Pyranose sugars have a cyclic structure based on the pyran ring. A pentose is any five-carbon carbohydrate molecule. Cyclohexose is a made-up term and has no meaning.</p>
<p><strong>15. D:</strong> All of these compounds possess a nitrogen-containing ring in their structures, so all are nitrogen heterocycles.</p>
<p><strong>16. B:</strong> Friedel-Crafts reactions typically use a strong Lewis acid such as AlCl<span class=”style4″>3</span> or FeCl<span class=”style4″>3</span> as a catalyst. Hydrogen chloride is not effective as a catalyst for Friedel-Crafts reactions. Friedel-Crafts reactions are applicable to most aryl and aromatic compounds.</p>
<p><strong>17. A:</strong> The lone pair electrons on the N atom of an amide are stabilized by delocalization into the carbonyl group <img src=”images/pi.jpg” alt=”pi” width=”10″ height=”9″ />system. The orbital hybridization of the N atom is thus stabilized as <img src=”images/sp2.jpg” alt=”sp2″ width=”21″ height=”14″ /> <em>+ p</em>, which has trigonal planar geometry. Rotation about the amide C-N bond is restricted by the <em>p</em> orbital overlap.</p>
<p><strong>18. A: </strong>These are all long-chain molecules of repeating monomeric units; amino acids for proteins, and glucose for starches and celluloses. They are produced as normal products of biological and biochemical processes, and so are called biopolymers.</p>
<p><strong>19. C:</strong> In simple terms, the loss of electrons is oxidation, and the gain of electrons is reduction. In a redox reaction, one species is reduced and another is oxidized. There must always be one of each for a redox reaction to occur. One species will be the reducing agent and the other will be the oxidizing agent. They have an equal but opposite effect on each other in that the reducing agent gives up the same number of electrons as the oxidizing agent accepts. Since the reducing agent gives up electrons, it is oxidized, and similarly since the oxidizing agent accepts electrons, it is reduced.</p>
<p><strong>20. A:</strong> The R- and S- designations of stereochemistry refer to the absolute order of substituents about an asymmetric center. To determine the order, one visualizes the molecule as seen in the direction of the bond to the lowest priority substituent, which in this case was the C-H bond. The priorities of the remaining three substituent’s are assigned from highest to lowest using the same rules as for E- and Z- designations. Their order then determines the designation as R- for clockwise order and S- for counterclockwise order.<br />
The D- and L- designations refer to the direction in which the compound rotates the plane of plane-polarized light passing through the molecules, D- for dextrorotatory, or to the right, and L- for levorotatory, or to the left.<br />
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