Chapter 12 (4.1.2)

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons containing single C-C and C-H bonds, as sigma (σ) bonds.

Question 2

What are sigma bonds?

Answer 2

A bond formed by the overlap of one orbital from each bonding atom, consisting of 2 electrons + with the electron density centred around a line directly between the nuclei of the 2 atoms.

Question 3

What is the shape around a carbon atom in an alkane and why?

Answer 3

Tetrahedral, bond angle 109.5°

Each carbon is surrounded by 4 electron pairs in the 4 sigma bonds

Question 4

Can alkanes rotate?

Answer 4

σ-bonds act as axes around which the atoms can rotate freely, so shapes are not rigid

Question 5

What is the effect of chain length on boiling point?

Answer 5

• London forces act between molecules that are in close surface contact
• As chain length increases, molecules have a larger surface area, so more contact is possible between molecules
• London forces greater so more energy needed to overcome forces so higher boiling point

Question 6

Effect of branching on boiling point

Answer 6

• branched isomers have lower boiling points than straight-chain isomers
• fewer surface points of contact between molecules of branched alkanes, giving fewer London forces
• branches get in way and prevent molecules getting close together, decreasing IM forces and thus b.p. decreases

Question 7

Why do alkanes have a lack of reactivity?

Answer 7

• C-C and C-H σ-bonds are strong and have high bond enthalpy
• C-C bonds are non-polar
• similarity in electro negativity of C and H mean the C-H bond can be considered non-polar

Question 8

Complete combustion of alkanes

Answer 8

In a plentiful supply of oxygen, alkenes burn completely to produce water + carbon dioxide

Question 9

Why are alkanes good fuels?

Answer 9

• give out heat
• readily available
• easy to transport
• burn in plentiful supply of oxygen without releasing toxic products

Question 10

Incomplete combustion

Answer 10

In a limited supply of oxygen, H atoms in alkane are always oxidised to water, ur combustion of carbon may form toxic CO or carbon as soot. May occur in closed space such as car engine of faulty heating systems

Question 11

Risks of carbon monoxide

Answer 11

It’s a colourless, odourless and highly toxic gas which can cause nausea, headaches etc and with high exposure death

Question 12

Alkane reaction with halogens

Answer 12

• sunlight gives the high-energy ultraviolet radiation needed for the reaction to occur
• substitution of Cl and Br; F is too reactive and I is not reactive enough
• e.g. CH4 + Br2 —> CH3Br + HBr

Question 13

What is radical substitution?

Answer 13

How alkanes undergo halogenation

Question 14

Describe initiation

Answer 14

• halogen molecule absorbs UV radiation for bond fission
• halogen undergoes homolytic fission where covalent bond is broken
• produces 2 highly reactive halogen radicals
  Br-Br —> (UV) Br • + •Br

Question 15

Describe propagation (chain reaction)

Answer 15

1) CH4 + Br• —> •CH3 + HBr
2) •CH3 + Br2 —> CH3Br
It could be a continuous cycle if the halogen radical reacts with another alkane as in the first part to the chain reaction

Question 16

Describe termination

Answer 16

• 2 radicals collide + pair up their unpaired electrons to form a covalent bond forming a molecule
• both radicals removed from reaction mixture, stopping reaction
• multiple possible steps

Question 17

Why is radical substitution limited?

Answer 17

Because the synthesis forms a mixture of organic compounds

Question 18

How does further substitution limit radical substitution?

Answer 18

Halogen radicals can collide with haloalkane, substituting a further hydrogen atom. Further substitution can continue until all hydrogen atoms have been substituted, forming mixtures such as CH3Br, CH2Br2, CHBr3, CBr4 which is an inconvenience to separate.

Question 19

How does substitution at different positions in a carbon chain limit radical substitution?

Answer 19

If a carbon chain is longer, a mixture of mono-substituted isomers by substitution at different positions in the carbon chain is produced.