Alkenes

Question 1

What is the bond angle around the C=C bond?

Answer 1

120

Question 2

How is a pi bond formed?

Answer 2

•by the sideways overlap of two p oribtals on each carbon atom forming a pi bond above and below the plane of molecule

Question 3

What is a addition reaction?

Answer 3

Where two molecules react to produce one

Question 4

Reaction of alkenes with hydrogen?

Answer 4

• hydrogen adds across the double bond to form a saturated alkane
• nickel catalyst
• e.g ethene + H2 –> ethane

Question 5

Why do addition reactions happen?

Answer 5

• the double bond has plenty of electrons and is easily attacked by electrophiles

Question 6

How does the double bond behave like a nucleophile ?

Answer 6

• its attracted to places with not a lot of electrons

Question 7

How is magarine manufactured?

Answer 7

• by the hydrogenation of vegetable oil

Question 8

How do you test for the presence of a double bond?

Answer 8

• when an alkene e.g cyclohexane is bubbled through an orange solution of bromine at room temp the solution goes from Orange –> colourless

Question 9

What do reactions with hydrogen halides form?

Answer 9

• halogenalkanes

Question 10

When non symmetrical molecules and hydrogen halides react with alkenes, why do some give rise to two products?

Answer 10

• because the alkenes unsymmetrical

Question 11

Reaction with steam?

Answer 11

• steam can be added across double bond to form alcohols
• pressure : 60atm
• catalyst : phosphoric acid
• temperature : 600K

Question 12

Why do reaction with steam have a high atom economy?

Answer 12

• because there are no ware products

Question 13

Why are alkenes more reactive than alkanes?

Answer 13

• because they contain a high electron density in the double bond

Question 14

Explanation of electrophillic addition

e.g C2H4 + Br2 –> C2H4Br2

Answer 14

• as the bromine molecule approaches the ethene is becomes polarised by the high density of electrons present in the double bond. The pi bond breaks and an electron pair forms a new bond with the + charged bromine atom
• at the same time that the bond is formed between the carbon and bromine the Br-Br bond breaks (hetrolytic fission) to form a bromide ion and carbon cation
• carboncations are highly reactive and the bromide reacts rapidly with it. A covalent bond is formed between the carboncation and bromide ion
• in the mechanism, the polarised bromine molecule has behaved like an electrophile. The bromine is attracted to the double bond where it accepts a pair of electrons to form a new covalent bond