Alkenes

Question 1

What are the bond angles on ethene?

Answer 1

rougly 120

it is a planar (flat) molecule

Question 2

Why can’t the double bond rotate?

Answer 2

there is no p orbital (which contains a single electron)

on each carbon these two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond

this is called a pi orbital

Question 3

How do alkenes react differently?

Answer 3

the double bond enthalpy is almost twice that for the single

although they are more reactive

the C=C forms an electron-rich area in the molecule, which can easily be attacked by positively charged reagents

the reagents are called electrophiles - electron pair acceptor

most alkene reactions are electrophillic additions

Question 4

Name and outline the mechanism for the reaction of alkenes with hydrogen halides

explain each step

Answer 4

• bromine more EN - so it is polar
• electrophile is the partially positive hydrogeb
  
  • attracted to C=C bonds high electron density
• one of the pairs of electron from the C=C forms a bond with the H to from a carbocation
  
  • electrons in H-Br and drawn towards the Br
• bond breaks heterolytically
  
  • electrons form shared pair go to bromine atom beause it is more EN than a hudrogen, leaving a Br- ion
  • this attaches to the +vely charged carbocation with one of its electron pairs

Question 5

What is the positive inductive effect?

Answer 5

• alkyl groups, e.g. -CH3 or -C2H5, have a tendency to release electrons
• tends to stabilise the positive charge of the intermediate carbocation -
  
  • the more alkyl groups attached the positively charged carbon atoms, the more stable the carbocation is
  • the products of the reaction will tend to come from the more stable carbocation
    
    • more likely to exist as a carbocation

Question 6

What are the different types of carbocation? Which are the most stable?

Answer 6

Question 7

What are the possible carbocations that can be formed when propene reacts with HBr ? Which is most likely to be formed

Answer 7

Question 8

How do halogens react with alkenes?

Answer 8

the halogen atoms add across the double bond, halogens act as electrophiles

Question 9

How do the halogens act as electrophiles despite being non-polar?

Answer 9

• at any moment the molecule is likely to have an instantaneous dipole,
• the partially positive side is attracted to the electron-rich double bond
• they react with one of the carbons and leave a carbocation, which reacts with the halogen ion
• forms a dibromo alkane

Question 10

What is the test for a double bond?

Answer 10

the addition reaction is used to a test for a double bond bromine solution is added, turns reddish-brown two colourless if a alkene is formed

Question 11

what are the conditions for an alkene reaction with sulfuric acid? What type of reaction is it ?

Answer 11

exothermic reaction

electrophillic addition

Question 12

Draw the mechanism for the reaction of ethen with conc sulfuric acid? What is the product?

Answer 12

Question 13

How is ethanol produced from the raction with conc sulfuric acid?

Answer 13

• whenn water is added to the ethyl hydrogensuylfate product, sulfuric acid reforms
• acts as catalyst

Question 14

How do alkenes react with water?

Answer 14

• water adds across the double bond
• the reaction is used industrially to make alcohols and is carired out with steam at a suitable pressure
• using a catalyst such as phosphoric acid (H₃PO₄)