C. Correct! A compound that consists only of hydrogen and carbon is termed a hydrocarbon.

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1 Organic Chemistry - Problem Drill 06: Alkanes and Cycloalkanes No. 1 of What is a hydrocarbon? (A) A compound that consists of only carbon. (B) A compound that consists of nonmetals. (C) A compound that consists of hydrogen and carbon. (D) A compound that consists of carbon, hydrogen, and oxygen. (E) A compound that consists of only carbon and oxygen. There are a few examples of substances that consist of only carbon (diamond is one of them), however, hydrocarbons are comprised of a little more than just carbon. Go back and review the definitions found in the tutorial for this section. Nonmetal is a rather broad term. ydrocarbons consist of specific element(s). Go back and review the definitions found in the tutorial for this section. C. Correct! A compound that consists only of hydrogen and carbon is termed a hydrocarbon. Oxygen is not a component of hydrocarbons. Go back and review the definitions found in the tutorial for this section. Oxygen is not a component of hydrocarbons. Go back and review the definitions found in the tutorial for this section. (1) Recall the definition of hydrocarbon. A hydrocarbon is defined as a compound that is composed entirely of just hydrogen and carbon. (2) Read each statement carefully and pick the correct answer. Therefore, the correct answer is (C).

2 No. 2 of Which of the following is an incorrect statement of the physical properties of alkanes and cycloalkanes? (A) Alkanes and cycloalkanes are nonpolar. (B) Alkanes and cycloalkanes do not have intermolecular hydrogen bonding. (C) Alkanes and cycloalkanes are hydrophobic. (D) With alkanes and cycloalkanes, their boiling points increase with increasing molecular weight while their melting points decrease with increasing molecular weight. (E) Alkanes and cycloalkanes are less dense than water. Alkanes and cycloalkanes are hydrocarbons. Since they consist of only hydrogen and carbon, there are no electronegative atoms in the molecules that would introduce dipoles. They are very nonpolar. In order for a molecule to be able to participate in hydrogen bonding, it would need a hydrogen bonded to an oxygen, nitrogen or fluoride. Alkanes and cycloalkanes, being hydrocarbons, consist of only hydrogen and carbon. ydrophobic is defined as water hating. One of the properties of hydrocarbons is that they are hydrophobic. D. Correct! While the boiling points of alkanes and cycloalkanes do increase with increasing molecular weight, their melting points do not decrease with increasing molecular weight. Their melting points also increase with increasing molecular weight. Alkanes and cycloalkanes are less dense than water and will float on top of a layer of water. Often this effect is seen when motor oil or gasoline is spilled near a puddle of water. (1) Recall the physical properties of alkanes and cycloalkanes. Being hydrocarbons, alkanes and cycloalkanes are nonpolar, hydrophobic, and incapable of participating in hydrogen bonding. They are typically less dense than water and their boiling and melting points increase with increasing molecular weight. (2) Read each statement and determine which statement is incorrect. Therefore, the correct answer is (D).

3 No. 3 of Which conformation of pentane appears below? C 2 C 3 C 3 (A) anti (B) gauche (C) eclipsed (D) totally eclipsed (E) None of the above. In an anti conformer, the largest groups are the furthest apart as possible 180 degrees. The dihedral angle between the methyl and ethyl groups above is considerably less than 180 degrees. B. Correct! In a gauche conformer, the two largest groups are 60 degrees apart. This is the case in the above conformer. In an eclipsed conformer, all the groups on the front carbon line up with groups on the back carbon (the dihedral angle is 0 degrees). The above conformer is staggered so it can not be eclipsed. In a totally eclipsed conformer, all the largest groups line up and block each other (the dihedral angle is 0 degrees). The above conformer is staggered so it can not be totally eclipsed. The correct answer is listed in the above answers. Go back and review the different types of conformers for alkane chains like pentane. (1) Recall the different type of conformers. You learned about 4 different conformers in the tutorial. There was the anti conformer, the eclipsed conformer, the totally eclipsed conformer, and the gauche conformer. In an anti conformer, the largest groups are 180 degrees apart. In an eclipsed conformer, all the groups on the front carbon line up with groups on the back carbon. In a totally eclipsed conformer, all the largest groups line up and block each other. In a gauche conformer, the two largest groups are 60 degrees apart. (2) Look at the conformer above and determine the angle between the two largest groups. The angle between the methyl and ethyl groups is 60 degrees, so the conformer must be gauche. Therefore, the correct answer is (B).

4 No. 4 of Which cycloalkane below is most likely to undergo a ring opening reaction due to ring strain? I II III IV (A) I (B) II (C) III (D) IV (E) Both III and IV. A. Correct! Cyclopropane, having a carbon-carbon bond angle of 60 degrees, is under considerable strain. This strain increases the likelihood the compound will undergo a ring opening reaction. aving a carbon-carbon bond angle of 90 degrees, cyclobutane does experience some strain and is susceptible to ring opening reactions. owever, of this group, it is not the most likely to undergo a ring opening reaction. Cyclopentane has a carbon-carbon bond angle of 108 degrees. It does not experience great strain from this bond angle so it is not the most likely to undergo a ring opening reaction. Cyclohexane has a carbon-carbon bond angle of degrees. It does not experience great strain from this bond angle so it is not the most likely to undergo a ring opening reaction. Cyclopentane and cyclohexane have bond angles close to or at degrees resulting in very little ring strain. These compounds would not undergo a ring opening reaction due to ring strain. (1) Recall the carbon-carbon bond angles of the most common cycloalkanes. Cyclopropane: 60 degrees Cyclobutane: 90 degrees Cyclopentane: 108 degrees Cyclohexane: degrees The further these bond angles are from the ideal of degrees, the greater the strain in the bonds. As this strain increases, the likelihood increases that the ring will undergo a ring opening reaction. (2) Look at the available choices and pick the ring with the bond angle furthest from the ideal. Of these choices, cyclopropane is the most likely to undergo a ring opening reaction. Therefore, the correct answer is (A).

5 No. 5 of Which cyclohexane conformer below is least stable? I II III IV (A) I (B) II (C) III (D) IV (E) II and III are equally least stable. This is one of chair conformers of cyclohexane. The chair is the most stable conformer as all the hydrogens are as far away as possible from each other reducing steric interactions. This is the twist boat conformer. While not the most stable conformer, it is not the least stable. Which conformer has the most steric interactions between its hydrogens? C. Correct! This is the boat conformer and it is the least stable conformer due to steric interactions between the hydrogens on the ring. This is one of chair conformers of cyclohexane. The chair is the most stable conformer as all the hydrogens are as far away as possible from each other reducing steric interactions. The twist boat conformer is more stable than the boat. The boat has flagpole interactions that greatly destabilize it. By twisting the two top points of the ring away from each other, the twist boat conformer avoids these destabilizing interactions. (1) Recall the different conformers of cyclohexane. There are two chair conformations possible for cyclohexane. These conformers are the most stable since the substituents on the ring are the furthest away from each other as possible. The boat conformer is the least stable due to the flagpole hydrogen interactions. The hydrogens on the tips of the ring are forced close together and experience repulsive strain. Flagpole hydrogens The twist boat conformer is twisted to avoid these flagpole interactions. While this twisting alleviates some of the strain making it more stable than the boat conformer, the twist boat is still less stable than the chair conformer. (2) Identify each conformer as a chair, twist boat or boat. Choose the least stable conformer. Of the conformers shown, III, the boat conformer, is the least stable conformer. Therefore, the correct answer is (C).

6 No. 6 of What reaction of alkanes yields a mixture of smaller alkanes and alkenes? (A) Combustion (B) Radical alogenation (C) Elimination (D) Reduction (E) Cracking Combustion converts alkanes to carbon dioxide and water, not other alkanes and alkenes. Go back and review the reactions of alkanes in the tutorial. Radical halogenation coverts alkanes into alkyl halides, not other alkanes and alkenes. Go back and review the reactions of alkanes in the tutorial. Alkanes do not under elimination reactions. Go back and review the reactions of alkanes in the tutorial. Alkanes are already in their most reduced form. Go back and review the reactions of alkanes in the tutorial. E. Correct! Cracking of hydrocarbons at high temperatures yields a mixture of smaller alkanes and alkenes. (1) Recall the reactions of alkanes discussed in the tutorial. We learned in the tutorial that alkanes undergo three types of reactions: combustion, halogenation, and cracking. Combustion converts alkanes to carbon dioxide and water in the presence of oxygen. alogenation of alkanes occurs with a halogen in the presence of heat or light to give alkyl halides. And lastly, cracking occurs at high temperatures to give smaller alkanes and alkenes. (2) Read each statement carefully and choose the one that is correct. Therefore, the correct answer is (E).

7 No. 7 of What is the correct IUPAC name for the compound below? (A) 1-ethyl-4-methyl-3-propylcycloheptane (B) 4-ethyl-1-methyl-2-propylcycloheptane (C) 3-(2-methyl-6-ethylcycloheptyl)propane (D) 6-ethyl-2-methyl-1-propylcycloheptane (E) 1-(2-methyl-6-ethylcycloheptyl)propane The cycloalkane should be numbered to give the lowest possible combination of numbers to the substituents. Review your numbering of the cycloalkane and see if you can find an even lower combination of numbers. B. Correct! The molecule is correctly named as a cycloalkane since the ring contains more carbons than any of the straight chains. By numbering the carbon attached to the methyl group as carbon 1, you obtain the lowest combination of numbers by continuing to number counterclockwise around the ring Since there are more carbons in the ring then in the longest straight chain, the molecule should be named as a cycloalkane. Go back and review the IUPAC rules for naming cycloalkanes. The cycloalkane should be numbered to give the lowest possible combination of numbers to the substituents. Review your numbering of the cycloalkane and see if you can find an even lower combination of numbers. Since there are more carbons in the ring then in the longest straight chain, the molecule should be named as a cycloalkane. Go back and review the IUPAC rules for naming cycloalkanes. (1) Find the longest carbon chain and the main functional group in the molecule to determine the parent The longest carbon chain is the 7 carbon ring. There is a three carbon chain, however, if the ring contains more carbons than any of the chains, the molecule is named as a cycloalkane. Since there are 7 carbons in the ring, it is a cycloheptane. (2) Determine the substituents and their correct location numbers. There are three substituents: a methyl group, an ethyl group, and a propyl group. The cycloalkane is numbered to give the lowest possible combination of numbers to the substituents. By numbering the carbon attached to the methyl group as carbon 1, you obtain the lowest combination of numbers by continuing to number counterclockwise around the ring. The propyl group is on carbon 2 and the ethyl group is on carbon 4. (3) Put the substituents in alphabetical order (ignoring any numerical prefixes) and place in front of the parent name. 4-ethyl-1-methyl-2-propylcycloheptane Therefore, the correct answer is (B).

8 No. 8 of What is the common name for the alkane below? C 3 C 3 C 3 (A) sec-butyl cyclopentane (B) isobutyl cyclopentane (C) neopentyl cyclopentane (D) tert-butyl cyclopentane (E) n-butyl cyclopentane In a sec-butyl group, the cyclopentane would be attached to the sec-butyl group via a secondary carbon. Go back and review the different common nomenclature prefixes. Isobutyl indicates a structural motif where two methyl groups are attached to a C group. ere, three methyl groups are attached to a center carbon. Neopentyl indicates a structural motif where three carbons are attached to a quaternary carbon and in turn the quaternary carbon is attached to a primary carbon. Five carbons total are in a neopentyl group. ere three methyl groups are attached to a center carbon. Go back and review the common nomenclature prefixes. D. Correct! The cyclopentane ring is attached to a tert-butyl group. A normal chain describes a straight chain of carbons. owever, this structure contains a cycloalkane attached to a T-like structure. Go back and review the common nomenclature prefixes. (1) When asked for common nomenclature, first determine the type of compound the given molecule is. In this particular case, we have been given an alkane chain attached to a cycloalkane. (2) Recall the common nomenclature rules for naming hydrocarbons. In alkane common nomenclature, use all the carbons in the molecule to determine the alkyl name if an acyclic alkane. If a cycloalkane is involved, name the acyclic portion first followed by the name of the cycloalkane. Use common prefixes as necessary. (3) Look for common structural motifs. Look to see if the structure is a straight chain with no branching or if there is branching, determine where the branching occurs. You have approximately five choices to describe the structure of the alkyl group: (I) (II) (III) (IV) (V) n- is used if the chain is straight and the functional group is on one end of the chain. Iso- is used if the chain contains two methyl groups attached to a C with the functional group on the opposite end of the chain. Sec- is used if the functional group is attached to the secondary carbon of the chain. Tert- is used if the functional group is attached to the tertiary carbon of the chain. Neo- is used if the functional group is attached to a carbon that has 4 other carbons attached to it. In the above structure, the acyclic portion is a tert-butyl group. It is attached to a cyclopentane ring. Since the ring contains more carbons than the acyclic portion, the compound is named as a cycloalkane. (4) Put the common alkane name together. The common name of the molecule is tert-butyl cyclopentane. Therefore, the correct answer is (D).

9 No. 9 of The following is a step from the radical bromination of ethane. What type of step is it? C 3 C 3 + Br C 3 C 2 + Br (A) Initiation (B) Propagation (C) Termination (D) Dehydrohalogenation (E) Carbocation formation In initiation steps, radicals are formed. The above step shows that while one radical is formed, another one is consumed. B. Correct! In propagation steps, one radical is consumed while another radical is formed. This is the first of two propagation steps in the radical bromination of ethane. In termination steps, radicals are consumed without new ones being made. The above step shows that while one radical is consumed, a new one is formed. Dehydrohalogenation is a different process than radical bromination. Go back and review the different steps involved in a radical halogenation reaction. No carbocations are formed in a radical process. The step above shows one radical is formed while another one is consumed. (1) Recall the different steps involved in radical halogenation reactions. There are three types of mechanistic steps in the radical halogenation of alkanes: initiation, propagation, and termination. In initiation steps, radicals are formed. In propagation steps, one radical is consumed while another radical is formed. In termination steps, radicals are consumed without new ones being made. (2) Compare the above step with these definitions. Determine what kind of mechanistic step it is. In the step shown, a bromine radical grabs a hydrogen from ethane. The bromine radical is consumed in the process. owever, a new radical, an ethyl radical, is formed. Therefore, this must be a propagation step. Therefore, the correct answer is (B).

10 No. 10 of Which of the following is not a termination step? (I) Br + Br Br Br (II) C 3 C 2 + Br C 3 C 2 Br (III) Br 2 2 Br (IV) All of the above are termination steps. (A) I (B) II (C) III (D) IV (E) None of the above. In I, two bromine radicals are consumed by forming diatomic bromine. This is a termination step. In II, an ethyl radical and a bromine radical are consumed by forming bromoethane. This is a termination step. C. Correct! In III, diatomic bromine is split into to bromine radicals. This is an initiation step, not a termination step. Not all the steps shown are termination steps. Go back and review the definition of a termination step and try to determine the correct answer. The correct answer can be found in the above answers. Go back and review what occurs during a termination step of a radical process. (1) Recall the different steps involved in radical halogenation reactions. There are three types of mechanistic steps in the radical halogenation of alkanes: initiation, propagation, and termination. In initiation steps, radicals are formed. In propagation steps, one radical is consumed while another radical is formed. In termination steps, radicals are consumed without new ones being made. (2) Determine if each choice meets the criteria of a termination step. Are all radicals consumed? In I and II, radicals are consumed and stable, neutral molecules are formed. No new radicals are formed. owever, in III, a stable, neutral molecule is split to form two bromine radicals. This is not a termination step but an initiation step. Therefore, the correct answer is (C).