More on Alkenes

Question 1

What happens in an electrophilic reaction?

Answer 1

In an ELECTROPHILIC ADDITION reaction, the alkene double bond opens up and the atoms are ADDED to the carbon atoms.

Question 2

Why does the electrophilic addition reactions happen?

Answer 2

Electrophilic addition reaction happens because the double bond has got plenty of ELECTRONS and is easily attacked by ELECTROPHILES.

Question 3

Why are electrophiles attracted to double bonds?

Answer 3

Electrophiles are electron-pair acceptors - they’re usually a bit short of electrons, so they’re attracted to areas where there are a lot of electrons about.

Question 4

Electrophiles include …

Answer 4

positively charged ions, like H+ and NO2+, and polar molecules (Since the positively charged atom is attracted to places with lots of electrons)

Question 5

Adding Hydrogen to C=C bonds include …

Answer 5

Alkanes.

Question 6

Ethene will react with Hydrogen gas in an addition reaction to …

Answer 6

produce ethane.

It needs a nickel catalyst and a temperature of 150 degrees.

Question 7

How is margarine made?

Answer 7

Margerine is made by ‘hydrogenating’ unsaturated vegetable oils. By removing some double bonds, you raise the melting point of the oil so that it becomes SOLID at room temeperature.

Question 8

Halogens react with Alkenes to form…

Answer 8

Dihalogenoalkanes.

Question 9

Halogens will react with alkenes to form DIHALOGENOALKANES - …

Answer 9

the halogens add across the double bond, and each of the carbon atoms ends up bonded to the halogen atom. It’s an ELECTROPHILIC ADDITION REACTION.

Question 10

When you shake an alkene with brown bromine water, the soolution quickly decoulourises. Why?

Answer 10

This is because bromine is added across the double bond to form a colourless DIBROMOALKANE.

So BROMINE WATER is used to test for the presence of carbon-carbon double bonds.

Question 11

Alcohols can be made by…

Answer 11

Steam hydration

Question 12

Alkenes can be hydrated by…

Answer 12

STEAM at 300 degrees and a pressure of 60-70atm. The reaction needs a solid PHOSPHORIC (V) ACID CATALYST.

Question 13

If you shake an alkene with ACIDIFIED POTASSIUM MANGANATE(VII) at room temperature, the…

Answer 13

purple solution is DECOLOURISED. You’ve OXIDISED the alkene and made a diol (an alcohol with two OH) groups.

Question 14

Alkenes also undergo ADDITION reactions with hydrogen halides - to form …

Answer 14

Halogenoalkanes

Question 15

Carbocations with more ALKYL GROUPS are more stable because …

Answer 15

the alkyl groups feed ELECTRONS TOWARDS THE POSITIVE CHARGE.
The MORE STABLE CARBOCATION is more likely to form.

Keywards: UNSYMMETRICAL alkene
Stable
Carbocation

Question 16

What is the Markownkoff’s rule?

Answer 16

The MAJOR PRODUCT from addition of hydrogen halide (HX) to an unsymmetrical alkene is the one where HYDROGEN adds to the carbon with the MOST HYDROGEN already attached.

Question 17

Alkenes will react with COLD CONCENTRATED SULFURIC ACID to form…

Answer 17

Alkyl hydrogen sulfates.

You can convert the alkyl hydrogen sulfates formed into ALCOHOLS by adding water and warming the reaction mixture.

Question 18

But-1-ene is an alkene. Alkenes contain at least one c=c double bond.

a) Describe how bromine water can be used to test for C=C double bonds.

Answer 18

Shake the alkene with bromine water. The solution goes from orange to colourless if a double bond is present.

Question 19

But-1-ene is an alkene. Alkenes contain at least one C=C double bond.

a) Describe how bromine water can be used to test for C=C double bonds.

Answer 19

Shake the alkene with bromine water. The solution goes from orange to colourless if a double bond is present.

Question 20

But-1-ene is an alkene. Alkenes contain at least one C=C double bond.

b) Name the reaction mechanism in the above test.

Answer 20

Electrophilic addition

Question 21

Hydrogen bromide will react with but-1-ene by this meachanism ,producing two isometric products

Explain why is this the major product for this reaction.

Answer 21

The carbo cation with the most attached alkyl groups is the most stable intermediate and is the most likely to form.