4.1.2 Alkanes

Question 1

Describe the bonding in alkanes

Answer 1

• saturated hydrocarbons
• contain single bonds
• contain sigma bonds - overlap of s-orbitals directly between the bonding atoms - sigma bond has free rotation and is very strong (region of high electron density so there are strong electrostatic forces of attraction between nucleus and electrons

Question 2

Describe the shape of alkanes

Answer 2

• tetrahedral shape around each carbon atom
• each bond angle is 109.5 degrees due to equal repulsion between bonding pairs

Question 3

Why does the boiling point of alkanes increase with chain length and decrease with branching?

Answer 3

Chain length
* molecules get larger so have a greater surface area
* so there is more surface contact between molecules
* so increased induced dipole-dipole interactions
* which require more energy to overcome, increasing the boiling point.
Branching
* more branches mean there are fewer surface points of contact and molecules can’t get as close
* so induced dipole-dipole interactions are reduced
* and require less energy to overcome

Question 4

Describe the reasons for the lack of reactivity in alkanes

Answer 4

• sigma bonds have a high bond enthalpy so are difficult to break
• C-C bond is non polar and electronegativity in C-H bond is so low it is essentially non-polar - LOW POLARITY - don’t attract nucleophiles or electrophiles

Question 5

Describe combustion in alkanes

Answer 5

burn completely in oxygen - release lots of energy so make good fuels
-incomplete combustion results in carbon monoxide or particulates

combustion always occurs in the gas state
larger alkanes release more energy per mole than shorter ones

Question 6

What conditions are needed for a halogen to react with an alkane and what is produced?

Answer 6

sunlight provides energy for reaction to occur - a substitution reaction occurs and a haloalkane is produced

Question 7

Describe the** initiation stage** of the bromination of an alkane

Answer 7

• covalent bond is broken using the UV rays from sunlight - photodissociation
• bond splits equally and each atom keeps one electron - homolytic fission
• two** highly reactive free radicals** are produced due to the unpaired electron

Question 8

Describe the propogation stage of the bromination of an alkane

Answer 8

• bromine radical reacts with the** C-H bond in a methane molecule** (attack) to form a **methyl radical **
• each methyl radical reacts with another bromine molecule - forms** bromomethane and bromine radical is regenerated**
• These steps continue in a chain reaction

Question 9

Describe the termination stage of the bromination of an alkane

Answer 9

• two radicals collide which forms a molecule where **all the electrons are paired **- the reaction stops as molecule is stable
  (there are multiple termination steps)

Question 10

What are the limitations of radical substitution in organic synthesis?
How can these limitations be tackled?

Answer 10

• mixture of products produced
• eg if there is** too much bromine,** a bromine radical can collide and react with the bromomethane and swap with a hydrogen atom which will form dibromomethane - this could happen again to form tribromomethane and tetrabromomethane
• OR RS can occur at any point along the carbon chain so a mixture of isomers can be produced

To tackle:
- add excess methane so a bromine radical is more likely to react with this than the bromomethane