Hydrocarbons

Question 1

What are alkanes?

Answer 1

Hydrocarbon molecules containing only sp3 hybridized carbon atoms (single bond)

• May be branched, unbranched or cyclic
• Unreactive, due to H saturation and no functional groups

Question 2

Give the ten common nomenclature roots for organic compounds

Answer 2

1C: meth
2C: eth
3C: prop
4C: but
5C: pent
6C: hex
7C: hept
8C: oct
9C: non
10C: dec

Question 3

Give the suffix for alkanes when:

• They are fragments (minus one H)
• Alkane compound

Give the prefixes for:

• C with one R group
• C with two R groups
• C with three R groups

Answer 3

fragments: yl (eg. propyl)
alkanes: ane (eg. methane)
C-R: normal/n
C-R2: secondary/sec (eg. sec-butyl)
C-R3: tertiary/tert/t (eg. tert-butyl).

Question 4

How many carbons are in gaseous alkanes? Liquid? Solid?

Answer 4

1-4 carbons: gas
5-17 carbons: liquid
17 and more carbons: solid

Question 5

What happens to the boiling point when the following are done?

• More carbons in straight chain alkanes
• Branching of alkanes

Answer 5

More carbons: higher boiling point

Branching: dramatic decrease in boiling point

Question 6

What types of solvents are alkanes soluble in?

Answer 6

• Nonpolar solvents (ie. benzene, carbon tetrachloride etc.)
• Not in aqueous solvents (they are hydrophobic due to low polarity and inability to hydrogen bond)
• Least dense of all organic compounds, and therefore generally float on water

Question 7

Describe complete and incomplete combustion of alkanes

Answer 7

Complete: Hydrocarbon converted to CO2 and H2O

Incomplete: the reaction gives other products when there is insufficient oxygen for complete combustion (eg. CO and soot/molecular C).

Question 8

What are the two most common reaction types with alkanes?

Answer 8

Combustion and free radical substitution reactions with halogens and uv/heat

Question 9

How does electron shell repulsion (ESR) effect cyclic molecules?

Answer 9

Substituents on cyclic carbon rings are more stable as equatorial (same plane as ring, equivalent to anti conformation), especially larger substituents (eg. t-butyl or a phenyl).

Question 10

Which of the following alkanes has the lowest heat of combustion per CH2 group?

1. Cyclopropane
2. Cyclobutane
3. Cyclohexane
4. Cycloheptane

Answer 10

3. Cyclohexane

The potential energy of a cyclohexane is a function of its torsional, steric, and ring strains. In order to make comparisons between cycloalkanes with different number of methylene units, divide the heats of combustion by the number of methylene units in the cycloalkane to get the potential energy per methylene.

The potential energy of cyclohexane per methylene is at a minimum when compared to the other cycloalkanes, because it can adopt a conformation that minimizes the torsional, steric and ring strains (chair). When it undergoes combustion, cyclohexane yields less heat per methylene than the other cycloalkanes, because it is initially at a lower potential energy per methylene.

Question 11

Does steric hindrance effect nucleophilic reactions?

Answer 11

Yes. Some compounds (especially aromatic ones) can express strong basicity and have functional groups that are normally very good nucleophiles. However, if they are sterically hindered, they will not be good nucleophiles.

Question 12

Which of the following alkyl halides can be used in an acetoacetic ester synthesis to make 4-methyl-2-pentanone?

A. Methyl chloride
B. Ethyl bromide
C. Propyl chloride
D. Isopropyl iodide

Answer 12

D. Isopropyl iodide

Isopropyl iodide can be used to make 4-methyl-2-pentanone.

An acetoacetic ester synthesis involves attaching an R group between the two carbonyl carbons (alkylation) before saponification (cleaving of ester), acidification and pyrolyzed (strongly heated), resulting in the removal of the ester side of the ester, leaving a methyl ketone bound to the new R group and a carboxylic acid.

Question 13

Why is a different base often used for a second alkylation of acetoacetic ester?

Answer 13

Because the active hydrogen of the monoalkylated product is less acidic, making a stronger base more effective the second time around.

The active hydrogen of the monoalkylated product is less acidic. Thus a stronger base may be needed to abstract this proton.

Question 14

The most stable substituted 6 carbon cyclic molecules are: ? (2)

Answer 14

1. The largest groups are always equatorial, as these keep charges maximally apart
2. The only thing that can overrule this is steric hindrance

Question 15

In any reaction, a symmetrical intermediate is assumed to form a ___ ratio of products.

Answer 15

A equal amount of each product.

If a mechanism has a symmetric intermediate compound, but the amount of products vary largely, the mechanism can most likely be rejected.

Question 16

What is a bimolecular reaction? How would you expect a change in concentrations to affect the reaction rate?

Answer 16

Bimolecular reactions involve two mechanisms acting on two molecules at the same time. An example would be a beta hydrogen atom being removed by a base in concert with the departure of a leaving group (bimolecular elimination reaction/E2).

Reaction rate is directly dependent to the concentrations of both reactants involved.

Question 17

What is Saytseff’s rule on alkenes?

Answer 17

The internal alkene (eg. C-C=C-C) is more stable, and without other kinetic factors getting involved, will predominate over terminal alkenes (eg. C-C-C=C).

This is important to note when looking at elimination reactions. Intermediary cations can rearrange to form internal and terminal alkenes, but internal alkenes are favoured.