Chapter 12 - Alkanes Flashcards

1
Q

Why are alkanes so unreactive?

A

Because they don’t have a functional group, which are areas of an organic compound that are susceptible to reactivity or attack.
As a result, all covalent sigma bonding between C-C and C-H are all NON-POLAR.
Also, the C-C and C-H sigma bonds are strong.

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2
Q

Why are organic compounds with functional groups susceptible to reactions?

A

The bond between Carbon and the 1st atom of the functional group is polar (because of a change of Electronegativity values). Therefore, they are a lot easier to break.

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3
Q

Alkanes can participate in what

A

Combustion and halogenation.

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4
Q

When can alkanes undergo complete combustion

A

Only if there is a sufficient supply of oxygen. As a result, carbon dioxide, water and heat and light are produced.

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5
Q

What is the pattern for complete combustion

A

For each CH2 added, you need 1 and a half supply of oxygen.

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6
Q

When can alkanes undergo incomplete combustion

A

when there is a limited supply of oxygen. it produces: carbon monoxide, water and carbon and heat.

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7
Q

why would there be a limited supply of oxygen

A

The reaction is happening in a closed space or a badly ventilated space.

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8
Q

How are alkanes extracted from crude oil?

A

By fractional distillation and at oil refineries.

  1. The crude oil is heated.
  2. A temperature gradient is established within the column.
  3. Hydrocarbon vapours evaporate and rise up the column.
  4. The vapours reach a height in the column where the temperature is less than its boiling point.
  5. Vapours cool and condense into a liquid and are collected in separate tanks.
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9
Q

As branching increases, how does this affect the molecule’s boiling point?

A

Increased branching means there are less contact points for London forces to act upon. The total strength of attraction between alkane molecules decreases. Hence, the amount of energy needed to break the forces also decreases.

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10
Q

As you increase the number of carbon atoms, how does this affect the boiling point?

A

If you increase the number of carbon atoms, you increase the number of contact points for London forces to act upon and therefore the strength of attraction between alkane molecules also increases. Therefore, the amount of energy and heat needed to break these forces also increases.

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11
Q

What is a halogenation reaction

A

It can undergo these reactions in the presence of UV radiation because it can give it the energy to start the reaction. It happens through radical substitution.

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12
Q

Radical substitution stages

A

I : Initiation
P: Propagation
T : Termination

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13
Q

Explain the difference between a halogen and a halide

A

A halogen is a molecule or a element from group 7, but a halide is an ion of a molecule from group 7.

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14
Q

Explain initiation

A

Halogen molecule absorbs the UV radiation (you need to write this above the arrow).
Halogen molecule undergoes homolytic fission, to produce 2 highly reactive halogen radicals.

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15
Q

Explain propagation

A

This is a 2 step chain reaction.
1. Halogen radical attacks Alkane H atom.
Producing an alkane radical and a hydrogen halide compound.
2. Alkane radical attacks another halogen molecule, producing a haloalkane compound and a new halogen radical.

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16
Q

Propagation example

A

P1: C2H6 + Cl* –> C2H5 + HCl
P2: C2H5 + Cl2 –> C2H5Cl + Cl*

17
Q

Explain termination

A

Where 2 radicals run into each other, pair up their unpaired electron to form a covalent bond.
As a result, stable molecules are formed (by sigma bonds) and radicals are removed from the reaction mixture.

18
Q

Termination example

A

T1 : Cl* + Cl* –> Cl2
T2 : Cl* + C2H5 –> C2H5Cl
T3 : C2H5 + C2H5 –> C4H10

19
Q

Why is it hard to make only 1 product by halogenation?

A

Because poly/multi substitutions are possible. A C-H bond in any product can be attacked by another cl* radical.

20
Q

What are the bonds in alkanes

A

Single covalent bonds, called sigma bonds

21
Q

What is the shape of alkanes

A

Each carbon atom has 4 electron pairs.

Repulsion between the electron pairs creates a tetrahedral arrangement with a 109.5 degree angle.

22
Q

General formula for Alkanes

A

Cn H2n+2

23
Q

What can the sigma bonds act as in alkanes

A

They can act as axes around which the atoms can roam freely. So the shapes aren’t rigid.