DF boq: Organic bonding; electrophilic addition of alkenes

Question 1

Define “electrophile”.

Answer 1

A cation or molecule with a partially positive (δ+) charge which may be attracted to a negatively charged region, and accept a lone pair of electrons to form a covalent bond.

Question 2

What is the name given to a single bond?

Answer 2

σ bond

sigma bond

Question 3

What does a double bond consist of?

Draw one.

Answer 3

• a σ bond
• a π bond

Question 4

How many electrons are in a double bond?

Answer 4

4

Question 5

How many σ-bonds and π-bonds are in the molecule (CH₃)₂CO?

Answer 5

9 σ-bonds

1 π-bond

Question 6

Why are double bonds less than twice as strong as single bonds?

Answer 6

• Single bond = σ bond
• Double bond = σ bond + π bond
• π bond weaker than σ because:
  
  • e^- in π bonds don’t attract nuclei as strongly, since further away
  • e^- in double bond repel each other
• So σ bond + π bond weaker than 2 x σ bond

Question 7

Explain, in terms of bonding, why alkenes are more reactive than alkanes.

Answer 7

• Alkenes have C=C bonds: 1 σ bond, 1 π bond
• π orbitals on C=C bonds are on the edges + more exposed so more reactive

Question 8

Define:

• Heterolytic fission (heterolysis)
• Homolytic fission (homolysis)

Answer 8

• Breakage of covalent bond where one species takes both bonding electrons (if molecule was neutral, anion + cation form)
• Breakage of covalent bond where each species takes one bonding electron (if molecule was neutral, 2 radicals form)

Question 9

Describe the test for unsaturation, and state what would be seen as a positive result.

Answer 9

• Bubble gas through bromine water
• Positive result: red-brown bromine decolourises

Question 10

What is an addition reaction?

Answer 10

One where 2 or more molecules react to form a single, larger molecule.

Question 11

• What mechanism occurs when ethene reacts with bromine?
• Draw the mechanism
• Describe what occurs during each step

Answer 11

Electrophilic addition

• Double bond on ethene induces instantaneous dipole on (polarises) Br₂
• Double bond attacks δ+ Br, donating a pair of electrons to form a covalent bond with it
• Pair of electrons in Br-Br bond move to other Br, forming Br^-
• Carbon which did not bond with Br becomes a carbocation
• Br^- attacks carbocation, donating a pair of electrons to form a covalent bond with it

Question 12

• What products are formed when hex-1-ene and bromine react in the presence of chloride ions?
• How is this evidence for the 2-step mechanism of electrophilic addition via a carbocation?

Answer 12

• ​​1,2-dibromohexane + 1-bromo-2-chlorohexane
• If mechanism were one-step, only ​​1,2-dibromohexane would form
• Formation of 1-bromo-2-chlorohexane proves mechanism must be two-step, involving an intermediate with a carbocation, which may be attacked by any anion present

Question 13

The test for unsaturation can involve shaking an alkene with bromine water, rather than pure bromine.

Explain the effect this has on the products of the reaction.

Answer 13

• Pure bromine would mean only a dibromoalkane would form
• Water molecules have lone pairs of electrons as well as bromide ions, so they compete as nucleophiles attacking intermediate carbocation
• Dibromoalkane and bromoalcohol form (proportions depend on concentration of Br₂(aq))

Question 14

Draw the mechanism for the formation of a bromoalcohol from the bromoethane cation.

Answer 14

Question 15

What mechanism occurs when ethene reacts with HBr, dissolved in a polar solvent, at room temperature?

Answer 15

Electrophilic addition

Question 16

What kind of reaction occurs when ethene reacts with gaseous HBr?

Answer 16

Radical addition (no ions involved)

Question 17

• What mechanism occurs when alkenes react with water?
• What is the name of the reaction that occurs?
• What is produced?
• What conditions are required?

Answer 17

• Electrophilic addition
• Hydration (addition of water across double bond)
• Alcohols
• H₃PO₄, steam, heat, pressure OR conc H₂SO₄ then water

Question 18

Draw the mechanism for the manufacture of ethanol using a phosphoric acid catalyst.

Answer 18

Question 19

• State the mechanism which occurs during the first step of the production of ethanol, catalysed by concentrated sulfuric acid.
• Draw the mechanism for the first step.
• Give the equation for the second step.

Answer 19

• Electrophilic addition

In step 2, the SO₃ vacates to join the H₂O molecule

Question 20

• What mechanism occurs when alkenes react with hydrogen?
• What is the name of the reaction that occurs?
• What is produced?
• What conditions are required?

Answer 20

• Electrophilic addition
• Hydrogenation
• Alkanes
• Ni, heat, pressure OR Pt, RTP

Question 21

Which of the following cannot be formed from ethene via a single synthetic step (i.e. single mechanism)?

1. CH₂BrCH₂Br
2. CH₃CHBr₂
3. CH₃CH₂OH
4. CH₃CH₂Br

Answer 21

2

1, 3 & 4 all have 2 species added across double bond: electrophilic addition. B has 2 bromines on one carbon, which can’t occur in one step.

Question 22

A compound that can be made in one synthetic step (mechanism) from pent-1-ene is:

1. 1,2-dibromopent-1-ene
2. 2-bromopentane
3. Pentan-3-ol
4. 2,3-dibromopentane

Answer 22

2

• 1: not viable since still unsaturated*
• 3: double bond joins carbons 1 and 2, not 3*
• 4: double bond joins carbons 1 and 2, not 3*