Alkanes

Question 1

What are alkanes?

Answer 1

• Saturated hydrocarbons containing only carbon and hydrogen atoms joined together by single covalent bonds
• Saturated hydrocarbons containing single C-C and C-H bonds as sigma bonds (overlap of orbitals directly between the bonding atoms) free rotation of the sigma bond

Question 2

Why does the chain length affect the boiling points of alkanes?

Answer 2

1. Different carbon chain length: increase chain length, increase boiling point
   - As chain length increases, the molecules have a larger surface area
   - So more surface contact is possible between the molecules
   - So more induced dipoles, and thus more London forces
   - So more energy is need to overcome the forces

Question 3

What is a sigma bond?

Answer 3

• Is the result of the overlap of two orbitals, one from each bonding atom
• Each carbon atom in an alkane has four sigma bonds

Question 4

What is the shape of an alkane?

Answer 4

1. Each carbon atom is surrounded by four electron pairs in four sigma bonds
2. Repulsion between these electron pairs results on a 3D tetrahedral arrangement around each carbon atom
3. Each bond angle is approximately 109.5 degrees

Question 5

How does branching affect the boiling point of alkanes?

Answer 5

1. Branching: More branching lower boiling point because
   -Less surface points of contact between molecules of the branched alkanes
   -Therefore there are fewer induced dipoles and thus fewer London forces
   -Therefore less energy is needed to overcome them
   +Also the shape of the molecules as the branches get in the way and prevent the branched molecules getting as close together as straight chain molecules, decreasing intermolecular forces further

Question 6

Why are alkanes not very reactive?

Answer 6

1. C-C and C-H sigma bonds are strong
2. C-C bonds are non polar
3. The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar

Question 7

What are the products of complete combustion of alkanes?

Answer 7

1. Alkane and oxygen goes to carbon dioxide and water

Question 8

What happens when alkanes are burnt in a limited supply of oxygen? Why is this dangerous

Answer 8

1. Incomplete combustion
2. Carbon monoxide is produced or soot
3. Combines irreversibly with haemoglobin so less oxygen transported

Question 9

What is it called when an alkane reacts with a halogen? What conditions are needed? What are the three steps?

Answer 9

1. Free radical substitution
2. UV light
3. Initiation, propagation and termination

Question 10

What are the limitations of radical subsection in organic synthesis?

Answer 10

1. Further substitution: and end up with a mixture of organic products e.g. Ch3BR, Ch2BR2, CHBR3, CHBR4
2. Subsitiution at different points in a carbon chain;
   - For methane, all four hydrogen atoms are bonded to the same carbon atoms, os only one monobromo compound CH3Br, is possible
   - With ethane, similarly only one mono substituted product CH2H5Br, is possible
   - If the carbon chain is longer, we will get a mixture of mono-substituted isomers by substitution at different points in the carbon chain. For example, pentane could form three monosubstituted isomers

Question 11

What is a saturated hydrocarbon?

Answer 11

Contains only carbon and hydrogen atoms bonded together by single covalent bonds

Question 12

Which of hexane and 3-menthyl pentane has a higher boiling points?

Answer 12

1. Hexane is a straight chain alkane and will have a higher boiling point, 3 methyl-pentane is branched
2. hexane molecules have more surface points of contact is between molecules. The London forces will be greater and so more energy is required to overcome the forces

Question 13

Which of hexane and pentane has a higher boiling point?

Answer 13

1. Hexane is a longer chain alkane than pentane and so has a higher boiling point
2. Hexane molecules have a greater surface area and there will be more surface contact between the molecules will be greater and so more energy is needed to overcome the forces