DF boq: Organic bonding; electrophilic addition of alkenes Flashcards
Define “electrophile”.
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.
What is the name given to a single bond?
σ bond
sigma bond
What does a double bond consist of?
Draw one.
- a σ bond
- a π bond
How many electrons are in a double bond?
4
How many σ-bonds and π-bonds are in the molecule (CH3)2CO?
9 σ-bonds
1 π-bond
Why are double bonds less than twice as strong as single bonds?
- 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
Explain, in terms of bonding, why alkenes are more reactive than alkanes.
- 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
Define:
- Heterolytic fission (heterolysis)
- Homolytic fission (homolysis)
- 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)
Describe the test for unsaturation, and state what would be seen as a positive result.
- Bubble gas through bromine water
- Positive result: red-brown bromine decolourises
What is an addition reaction?
One where 2 or more molecules react to form a single, larger molecule.
- What mechanism occurs when ethene reacts with bromine?
- Draw the mechanism
- Describe what occurs during each step
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
- 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?
- 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
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.
- 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))
Draw the mechanism for the formation of a bromoalcohol from the bromoethane cation.
What mechanism occurs when ethene reacts with HBr, dissolved in a polar solvent, at room temperature?
Electrophilic addition
Draw the mechanism for the manufacture of ethanol using a phosphoric acid catalyst.
- 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.
- Electrophilic addition
In step 2, the SO3 vacates to join the H2O molecule
