alkanes

Question 1

explain the bond angle and shape around each carbon atom in an alkane

Answer 1

each carbon atom is surrounded by 4 electrons in sigma bonds. repulsion between these electrons results in a tetrahedral shape and 109.5 bond angle around each carbon atom

Question 2

explain the variation in boiling points of alkanes in terms of chain length

Answer 2

as chain length increases, boiling point increases. this is because the london forces have more points of contact between molecules and so will be stronger, requiring more energy to overcome

Question 3

explain the variation in boiling points of alkanes in terms of branching

Answer 3

as branching increases, boiling point decreases. this is because the branches prevent the molecules from getting as closely together and also decreases the points of contact between them, making the london forces weaker (therefore less energy required to overcome)

Question 4

what is homolytic fission?

Answer 4

each bonding atom receives one electron from the bonding pair, forming two radicals

Question 5

what is heterolytic fission?

Answer 5

one bonding atom receives both electrons from the bonding pair

Question 6

define radical

Answer 6

a species with an unpaired electron

Question 7

what symbol is used to represent radicals in curly arrow mechanisms?

Answer 7

dots

Question 8

what does a curly arrow represent?

Answer 8

the movement of a pair of electrons, showing either heterolytic fission or the formation of a covalent bond

Question 9

why do alkanes have low reactivity?

Answer 9

C-C and C-H bonds are non polar, and have a high bond enthalpy

Question 10

what are the products of the complete combustion of an alkane?

Answer 10

carbon dioxide and water

Question 11

what are the products of the incomplete combustion of an alkane?

Answer 11

carbon monoxide (CO) or carbon as soot, and water

Question 12

why is carbon monoxide so dangerous?

Answer 12

it is colourless and odourless. it reacts irreversibly with haemoglobin to form carboxyhaemoglobin, which prevents the red blood cell from carrying oxygen around the body

Question 13

what do alkanes need to react with halogens (chlorine and bromine)?

Answer 13

UV (ultraviolet) radiation

Question 14

what are the limitations of free radical substitution in organic synthesis?

Answer 14

-further substitution
-substitution at different positions in a carbon chain e.g. pentane could form 3 different isomers (1-bromopentane, 2-bromopentane, 3-bromopentane)