MCAT Biology : Reactions with Hydrocarbons

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Reactions With Hydrocarbons

The most stable radical shown below is __________. The least stable radical shown below is __________.

Mcat_6

Possible Answers:

radical C . . . radical E

radical E . . . radical C

radical D . . . radical A

radical D . . . radical E

radical B . . . radical E

Correct answer:

radical C . . . radical E

Explanation:

Radical stability increases as carbon substitution increases. In addition, radicals in conjugation with double bonds via resonance are more stable than the corresponding non-conjugated radical.

In this case radicals B, C, and D are all tertiary radicals, but only radical C has additional stabilization from resonance. Radical C is therefore the most stable. Radical E is the least substituted of the five radicals, and is the least stable.

Example Question #2 : Reactions With Hydrocarbons

What intermediate is involved in the conversion of compound B to compound C?

Mcat_1

Possible Answers:

Tertiary carbanion

Secondary radical

Tertiary radical

Secondary carbocation

Tertiary carbocation

Correct answer:

Tertiary carbocation

Explanation:

The strong sulfuric acid protonates the hydroxyl group of compound B, resulting in the loss of water as a leaving group and the generation of a carbocation intermediate. Since this carbocation carbon is attached to three other carbons, this is a tertiary carbocation. It is bound to the phenyl substituent, a methyl group, and the branched carbon chain.

Example Question #34 : Organic Chemistry, Biochemistry, And Metabolism

The combustion of pentane with oxygen gas is an exothermic reaction that produces carbon dioxide and water as products. Is this a spontaneous reaction?

Possible Answers:

Yes, only at low temperatures

No, never

Yes, at all temperatures

Yes, only at high temperatures

Correct answer:

Yes, at all temperatures

Explanation:

A balanced equation of the combustion of pentane indicates that one mole of pentane reacts with eight moles of oxygen gas to produce five moles of carbon dioxide and six moles of water.

Because there are nine moles of reactant and eleven moles of product, entropy increases in this reaction. Exothermic reactions that increase entropy are favorable at all temperatures, as seen in the Gibb's free energy equation.

In our scenario, H is negative and S is positive.

Example Question #35 : Organic Chemistry, Biochemistry, And Metabolism

Which of the following statements concerning the combustion of cycloalkanes is false?

Possible Answers:

Oxygen is necessary in order for a combustion reaction to take place

The molar heat of combustion for cyclooctane is nearly double the molar heat of combustion for cyclobutane

The heat of combustion per  in cyclohexane is greater than the heat of combustion per  in cyclobutane

As the number of carbons in the ring increases, the molar heat of combustion increases

Correct answer:

The heat of combustion per  in cyclohexane is greater than the heat of combustion per  in cyclobutane

Explanation:

The heat of combustion for cycloalkanes can be quantified based on two factors: the number of carbons in the ring, and the amount of strain in the ring. Cyclohexane does in fact have a larger molar heat of combustion than cyclobutane. This is because there are more carbons in the ring; however, ring stability will determine the heat of combustion per  group in the ring. Even though cyclohexane has more carbons than cyclobutane, the heat of combustion per  group in cyclobutane will be greater compared to cyclohexane due to cyclobutane's ring strain. 

Example Question #36 : Organic Chemistry, Biochemistry, And Metabolism

Benzene

Benzene

 

1_3_5_hex-tri-ene

1,3,5-hexatriene

Which of the compounds shown above will have a greater heat of reduction (hydrogenation)?

Possible Answers:

More information is required

The compounds will have the same heat of reduction

Benzene

1,3,5-hexatriene

Correct answer:

1,3,5-hexatriene

Explanation:

The answer is 1,3,5-hexatriene. Chemicals that are more stable will give off less heat when they are reduced or hydrogenated. Although these two compounds contain the same number of pi bonds to be reduced, benzene is aromatic, and therefore is much more stable than the conjugated non-cyclic hexatriene. We would find that the heat of hydrogenation for hexatriene would be noticeably greater than that of benzene.

Example Question #6 : Reactions With Hydrocarbons

Alkenes A through D contain only carbon and hydrogen. They have the following heats of combustion at .

Alkene A:

Alkene B:

Alkene C:

Alkene D:

Rank these four alkenes in terms of their stability, with the least stable compound on the left, and the most stable compound on the right.

Possible Answers:

A < B < C < D

D < B < C < A

A < D < B < C

D < C < B < A

A < C < B < D

Correct answer:

D < B < C < A

Explanation:

A combustion reaction of any hydrocarbon yields the same products: carbon dioxide and water. The heat of combustion for the reaction shows how much energy is released as the hydrocarbon is converted to those products.

Since the products are the same for each alkene, any difference in heat of combustion will arise from differences in energy of the starting alkenes. Since the heat of combustion is negative, that means the reactants are higher in energy than the products, creating an exothermic reaction. Furthermore, the more negative the heat of combustion, the higher in energy the reactants are, and the less stable they are as well. Since compound D has the largest heat of combustion, it is highest in energy and therefore the least stable. By similar reasoning, B is the next highest in energy, followed by C, and then finally A, the most stable compound.

Example Question #1 : Free Radicals

Which of the following steps of free radical chlorination does not produce a free radical as a product?

Possible Answers:

Propagation

Halogenation

Termination

Initiation

Correct answer:

Termination

Explanation:

The three steps of a free radical chlorination reaction are, in order, initiation, propagation, and termination.

Free radicals are produced in the initiation and propagation steps. The termination steps combine any two free radicals formed in the reaction to produce a compound that has no unpaired electrons (free radicals).

Example Question #2 : Reactions With Hydrocarbons

Consider the following radical bromination reaction of propane.

Which of the following substances would NOT be a suitable radical initiator for this reaction?

Possible Answers:

 

Mcat11

  Azobisisobutyronitrile (or AIBN)

 

Benzoyl peroxide

tert-butyl peroxide

Diisopropyl ether

UV light

Correct answer:

Diisopropyl ether

Explanation:

For any radical reaction, a suitable radical initiator is required. This either involves a chemical that decomposes to produce a radical upon heating, or ultraviolet light. For the former, any peroxide is suitable. AIBN also works, as it extrudes nitrogen upon heating to product a radical.

An ether, by contrast, will not produce a radical upon heating and would therefore not be a suitable radical initiator.

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