Addition Reactions of Alkenes

Question 1

what is a C=C bond made up of

Answer 1

a sigma bond and a pi bond

Question 2

which bond is stronger and why

Answer 2

• the sigma bond is stronger

- because it is held more tightly between the carbon atoms

Question 3

with that information does that mean that a double bond is twice as strong as a single bond and why

Answer 3

• no, it is between 1 and 2 times stronger
• because every single bond is a sigma bond which is stronger than a pi bond
• so if you have a sigma bond and a pi bond like a C=C, it will be stronger but not twice as strong

Question 4

what do most reactions of alkenes generally involve in terms of the bonds

Answer 4

a double bond becoming a single bond

Question 5

what bonds change during an alkene addition reaction

Answer 5

• the sigma bond remains unchanged

- but the electrons in the pi bonds are used to form new bonds with an attacking molecule

Question 6

what is the product that is formed compared to an alkene if it only has single bonds

Answer 6

it is saturated

Question 7

why is the product more stable than the alkene

Answer 7

• because the product only has sigma bonds, not pi bonds

- meaning the overall bonds in the product are stronger

Question 8

what is the equation for the addition reaction between ethene and bromine

Answer 8

C2H4 + Br2 = C2H4Br2

Question 9

what is that reaction generally used for

Answer 9

the chemical test for the presence of C=C in a compound

Question 10

why is that reaction used as a chemical test

Answer 10

• because one of the reactants has a colour (bromine water)

- but the product is colourless

Question 11

what is the correct way to describe what happens to bromine in the reaction when it reacts and the product is colourless

Answer 11

you say the bromine has been decolorised

Question 12

what is hydrogenation

Answer 12

the addition reaction between an alkene and hydrogen

Question 13

what would be the equation for the hydrogenation of ethene

Answer 13

C2H4 + H2 = C2H6

Question 14

what will the the product of a hydrogenation reaction usually be and why

Answer 14

• an alkane
• because the product only has single bonds
• and has the general formula of CnH2n

Question 15

what are the two things that are required in order for a hydrogenation reaction to occur

Answer 15

• heat

- a nickel catalyst

Question 16

what are hydrogenation reactions practically used for

Answer 16

making margarine

Question 17

how is margarine formed from hydrogenation

Answer 17

• naturally occurring vegetable oils are unsaturated
• so they contain C=C bonds
• when they react with hydrogen, some of the double bonds become single bonds
• this changes the properties of the vegetable oil and converts it into a solid
• aka margarine

Question 18

what are monosaturated, polysaturated and satruated fats

Answer 18

• monosaturated fats are molecules that only have one C=C bons
• polysaturated fats are molecules that have more than one C=C bond
• saturated fats are molecules with no C=C bond

Question 19

what is a halogenation reaction

Answer 19

a reaction between an alkene and a halogen

Question 20

what are the products of most halogenation reactiond

Answer 20

dihalogenalkanes

Question 21

what would a reaction between ethene and bromine produce if one bromine were to be bonded with one carbon atom each

Answer 21

1,2-dibromoethane

Question 22

what is hydration

Answer 22

• adding water

- specifically, adding H and OH to the two atoms in a C=C bond

Question 23

how is a hydration react carried out

Answer 23

• heating an alkene with steam

- passing the mixture over a catalyst of phosphoric acid

Question 24

what would a reaction between ethene and water produce

Answer 24

ethanol

Question 25

what is the difference between hydrogenation and hydration reactions

Answer 25

• hydration reactions actually make useful products

- so they are therefore used in industry a lot more

Question 26

what are hydrogen halides often and what do they form in addition reactions

Answer 26

• they are hydrogen bromide or hydrogen chloride

- and they form halogenalkanes

Question 27

what is the structural formula and the name for the reaction between ethene and hydrogen bromide

Answer 27

• CH2=CH2 + H-Br = CH3-CH2Br

- bromoethane

Question 28

what is a diol

Answer 28

a compound containing two OH groups

Question 29

what is the oxidising agent in acid conditions and what acid is usually used

Answer 29

• potassium manganate (VII)

- sulfuric acid

Question 30

what is the summation of this reaction

Answer 30

• an oxidation reaction

- followed by an addition reaction

Question 31

how are two OH groups formed when ethene reacts with the solution and the potassium manganate

Answer 31

• the potassium manganate provides an oxygen atom (oxidising)
• the water in the solution provides another oxygen atom and two hydrogen atoms
• forming the addition of two OH groups across the double bond

Question 32

what is the structural formula for this reaction

Answer 32

CH2=CH2 + [O] + H2O = CH2OH-CH2OH

Question 33

what does the [O] represent

Answer 33

the oxygen supplied by the oxidising agent

Question 34

what is the name of the product

Answer 34

ethane-1,2-doil

Question 35

why can this reaction be used to distinguish between and alkene and alkane like bromine is

Answer 35

• potassium manganate is normally purple
• in this reaction it changes from that to colourless
• it only turns colourless if there is a double bond present for the reaction
• so it can only occur with alkenes