Alkanes

Question 1

What are alkanes?

Answer 1

Compounds that:

• Consist only of carbon and hydrogen
• Linked exclusively by single bonds (saturated compounds)

Methanes are the simplest possible alkane, while saturated fats and oils are much larger

Question 2

With alkanes, what is the general formula for a straight chain molecule?

Answer 2

CnH2n+2

Question 3

Are alkanes fully saturated (no double or triple bonds)?

Answer 3

Yes

Question 4

Do alkanes contain functional groups?

Answer 4

No

Question 5

Are alkanes chemically reactive?

Any exceptions?

Answer 5

No.

Except when exposed to hear or light

Question 6

With the nonmenclature of alkanes, what is used to determine the root of the compound?

Answer 6

The number of carbons in the longest chain:
C1 = Meth
C2 = Eth
C3 = Prop
C4 = But
C5 = Pent
C6 = Hex
C7 = Hept
C8 = Oct
C9 = Non
C10 = Dec

Question 7

What is an alkane fragment and how are they named?

Answer 7

An alkane minus one H atom.

The suffix “yl” is used

Question 8

How are alkane’s named?

Answer 8

The suffix “ane” is used

Question 9

What do the suffixes “yl” and “ane” denote?

Answer 9

Alkane fragments and alkanes, respectively

Question 10

What is a carbon with one R group called?

Answer 10

A primary carbon

Question 11

What is a carbon with two R groups called?

Answer 11

A secondary carbon

Question 12

What is a carbon with three R groups called?

Answer 12

A tertiary carbon

Question 13

How are cyclic alkanes named?

Answer 13

The same way as alkanes, except the prefix “cyclo” is added

Question 14

What is the shorthand for organic compounds?

Answer 14

A geometric figure where each corner represents a carbon.

Hydrogens need not be written, though it should be remembered that the number of hydrogens would exist such that the number of bonds at each carbon is four

Question 15

How are branched-chain alkanes named?

Answer 15

Longest straight chain is determined (highest number of carbons in a row). The groups attached to the main chain are numbered so as to achieve the lowest set of numbers. Groups are cited in alphabetical order. If a group appears more than once, prefixes di (2), tri (3), tetra (4) are used

Question 16

At room temperature and one atmosphere of pressure, what phase are straight chain alkanes?

Answer 16

1-4 carbons: gas.
5-17 carbons: liquid.
18 or more carbons: solid

This is because they are non polar molecules and have weak intermolecular forces

Question 17

What affect does the branching of alkanes have on the boiling point?

Answer 17

It dramatically decreases it

Question 18

In what type of solvents are alkanes soluble?

Answer 18

Non-polar.

Hydrophobic because of low polarity and inability to hydrogen bond

Question 19

How does the density of alkanes compare to that of other organic compounds?

Answer 19

They are the least dense -> thus petroleum which is a mixture of hydrocarbons rick in alkanes, floats on water

Question 20

What happens to alkanes in complete combustion?

Answer 20

Converted to CO2 and water.

If there is insufficient oxygen for complete combustion, the reaction gives other products such as carbon monoxide (CO) and soot (molecular carbon)

Question 21

Thermally, what type of reaction is the complete combustion of alkanes?
What is the formula?

Answer 21

Strongly exothermic.

CnH2n+2 + excess O2 —> nCO2 + (n+1)H2O

Question 22

What are the 3 steps of halogenation?

Answer 22

1. Initiation (involves a formation of free radicals)
2. Propagation (free radical begins a series of formations which form new free radicals)
3. Termination (these reactions end the radical propagation steps. Termination reactions destroy the free radicals through coupling)

Question 23

Use a formula to summarise radical substitution reactions with halogens.
What halogens may be used for X2?

Answer 23

RH + X2 + UV light (hf) or heat –> RX + HX.

F2, Cl2, Br2

Question 24

Chain propagation can destroy many organic compounds fairly quickly. How can this step of radical substitution reaction by inhibited?

Answer 24

By using a resonance stabilised free radical to “mop up”/terminate other destructive free radicals in the medium.

For example BHT is a resonance stabilised free radical added to packaging of many breakfast cereals in order to inhibit free radical destruction of the cereal (spoiling)

Question 25

What does the stability of a free radical depend on?

Answer 25

The ability of the compound to stabilise the unpaired electron.

This is analogous to stabilising a positively charged carbon (carbocation). Thus is both cases, a tertiary compound is more stable than a secondary which, is in turn more stable than a primary compound

Question 26

Cyclic alkanes are strained compounds, what does this mean?

What does it result from?

Answer 26

The ring strain results from the bending of bond angles in greater amounts than normal.

This strain causes cyclic compounds of 3 and 4 carbons to be unstable, and thus not often found in nature. The usual angle between bonds in an sp3 hybridised carbon is 109.5degrees (the normal tetrahedral angle)

Question 27

What is the expected angle in the following cyclic compounds:

• Cyclopropane
• Cyclobutane
• Cyclopentane
• Cyclohexane (in chair conformation)

Answer 27

• 60°
• 90°
• 108°
• 109.5°

Question 28

What is an easy way to determine the conformation of a molecule?

Answer 28

The way that would minimise electron shell repulsion (and therefore be at a lower energy level).

Question 29

What are equatorial hydrogens?
Axial?
How would these be arranged in a chair conformation of cyclohexane for example?

Answer 29

They are in the same plane as a ring of carbons.
They are perpendicular to a ring of carbons.
Maximally separated and staggered to minimise electron shell repulsion