[13.3] reactions of alkenes

Question 1

are alkenes more or less reactive than alkanes and why?

Answer 1

more reactive. due to the presence of the pi bond, which readily breaks so alkenes undergo addition reactions relatively easily

Question 2

what is an addition reaction?

Answer 2

when two reactants join together to form one product

Question 3

what addition reactions may alkenes undergo?

Answer 3

• hydrogen (nickel catalyst, 150° C)
• halogens
• halides
• steam (phosphoric acid catalyst, high temp + pressure)

Question 4

hydrogenation of alkenes

Answer 4

• when an alkenes is mixed with hydrogen and passed over a nickel catalyst at 423 K, an addition reaction takes place to form an alkane
• this addition reaction, in which hydrogen is added across a double bond, is known as hydrogenation

Question 5

halogenation of alkenes

Answer 5

alkenes undergo a rapid addition reaction with the halogens chlorine or bromine at room temperature

• when drawing the displayed formula, make sure the halogens are added to where the bond was broken

Question 6

testing for unsaturation

Answer 6

• when bromine water (orange) is added to an alkene, the solution is decolourised, indicating the presence of a C=C bond
• with alkanes, no colour change is observed
• this is because in an alkene, bromine adds across the double bond

Question 7

addition reactions of alkenes with hydrogen halides

Answer 7

• alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
• if the alkene is a gas, the reaction takes place when the two gases are mixed
• if the alkene is a liquid, hydrogen halide is bubbled through it
• there are two possible products of this reaction (topic 13.4)

Question 8

hydration reactions of alkenes

Answer 8

• alcohols are formed when alkenes react with steam in the presence of a phosphoric acid catalyst
• steam adds across the double bond (H and OH are added)
• there are two possible products, as with hydrogen halides