4.1.2 Alk_a_nes

Question 1

What demonstrates that alkanes lack reactivity

Answer 1

Alkanes are the main component of natural gas and crude oil. They are amongst the most stable organic compounds and their lack of reactivity has allowed crude oil to remain in the earth for millions of years.

Question 2

What are alkanes

Answer 2

alkanes are saturated hydrocarbons

Question 3

What is the general formula for Alkanes

Answer 3

C nH 2n+2

where n is the number of carbon atoms in a molecule

Question 4

What kind of bonds to saturated compounds form

Answer 4

Saturated compounds contain only single bonds. These are sigma bonds (𝜎)

Question 5

How are sigma (𝜎-) bonds formed

Answer 5

𝜎-bonds are formed by the “head on” overlap of orbitals

Question 6

describe a sigma bond in an alkane

Answer 6

The sigma bond is a type of covalent bond.
A sigma bond is positioned on a line directly between bonding atoms.
Each carbon atom in an alkane has 4 sigma bonds either C-H or C-C.
The electron density of the bond is concentrated between the two atoms.

Question 7

Explain the structure of an Alkane

Answer 7

Each carbon atom is surrounded by 4 electron pairs in 4 𝜎-bonds. Repulsion between these electron pairs results in a tetrahedral arrangement around each carbon. Therefore each bond angle is 109.5

Question 8

explain the structural movement of an alkane

Answer 8

The 𝜎-bond acts as an axes around which the atoms can rotate freely - so these shapes are not rigid.

eg. butane is shown as a zigzag but it can be rotated into a U shape

Question 9

How do the boiling points of alkanes change along the homologous series

Answer 9

The b.p. of alkanes increase moving up the homologous series
BUT the presence of branching also impacts b.p.

the higher ‘n’ the further up the homologous series

Question 10

What are the states for straight chain alkane isomers

Answer 10

CH4 to C4H10 = gases
C5 to C17 = liquids
C18+ = solids

Question 11

Why does boiling point increase up a homologous series of alkanes

Answer 11

The b.p. increases because there are more electrons in larger molecules so the London forces are stronger, therefore more energy is required to break them.

Question 12

How does branching in alkanes impact boiling point

Answer 12

In branching there are fewer surface points of contact between the molecules which gives fewer london forces so less energy is required to break them resulting in a lower b.p..
Additionally the branches get in the way of the molecules getting as close as possible so as a result decrease the london forces further

more branching → less surface area of contact → weaker london forces

Question 13

Why do alkanes not react with most common reagents

(the reasons for alkanes lack of reactivity are)

Answer 13

• C-C and C-H 𝜎-bonds are strong
• C-C bonds are non-polar
• the electronegativity of C and H are so similar that the C-H bond can be considered to be non-polar

Question 14

What does the complete combustion of alkanes produce

Answer 14

complete combustion produces CO₂ and H₂O

Question 15

What does the incomplete combustion of alkanes produce

Answer 15

Incomplete combustion produces CO and H₂O

Question 16

What dictates if the alkanes undergo complete or incomplete combustion

Answer 16

The availability of oxygen
If there is an excess of oxygen complete combustion occurs, if there is a lack of oxygen incomplete combustion occurs

Question 17

What is the difference between incomplete and complete combustion

other than product and abundacy of oxygen

Answer 17

• Complete combustion is more exothermic (produces more energy) than incomplete combustion
• Incomplete combustion produces CO which is toxic and odourless- restricts the ability of blood to carry oxygen

Question 18

what is the name of the reaction between alkanes and halogens

Answer 18

substitution reaction

Question 19

What halogens react with alkanes

Answer 19

You need to look at chlorine and bromine primarily and fluorine (the most reactive) is too reactive and would likely explode. Additionally iodine is not reactive enough and barely reacts with alkanes (not to say it never does)

Question 19

What is the mechanism for the bromination/ chlorination/ iodination of alkanes

Answer 19

radical substitution

Question 20

Describe initiation as the first step of radical substitution

Answer 20

The reaction starts when the covalent bond in a halogen molecule is broken due to the presence of UV light causing homolytic fission of the bond.
This results in the covalent bond breaking which splits the halogen molecule into halogen atoms called free radicals

e.g. Br-Br → Br’ + Br’

’ = the dot representative of the e⁻

Question 21

What are the steps of the mechanism of radical substitution

Answer 21

1. initiation
2. propagation
3. termination

Question 22

Describe propagation as the second step of radical substitution

Answer 22

This is the chain reaction.
A free radical reacts with a molecule to form a new molecule and a different free radical.
e.g. Cl’ + CH₄ → CH₃’ + HCl
The free radical produced can react with another molecule to form a new molecule and a different free radical.
e.g. CH₃’ + Cl₂ → CH₃Cl + Cl’

In each reaction a free radical reacts with a molecule to form a different molecule and a different free radical. This creates a chain reaction - each radical produces another. This will continue until termination

Question 23

Describe termination as the third step of radical substitution

Answer 23

Sometimes free radicals react with each other to form a molecule. This removes free radicals from the reaction stopping the chain.
e.g. CH₃’ + Cl’ → CH₃Cl
Making the overall reaction CH₄ + Cl₂ → CH₃Cl + HCl

Question 24

What is a limitation of radical substitution in organic synthesis

Answer 24

It produces a mixture of products
(especially as each product can then form a different one which can form another and so on )
This makes the yeild of the desired product low

The reaction require UV light so cannot happen in the dark - idk if this is a limitation but its a point

Question 25

Where can radical substitution happen in alkanes

Answer 25

Radical substitution can happen at any position in the carbon chain of more complicated alkanes