DF boq: Organic bonding; electrophilic addition of alkenes Flashcards

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1
Q

Define “electrophile”.

A

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.

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2
Q

What is the name given to a single bond?

A

σ bond

sigma bond

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3
Q

What does a double bond consist of?

Draw one.

A
  • a σ bond
  • a π bond
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4
Q

How many electrons are in a double bond?

A

4

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5
Q

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

A

9 σ-bonds

1 π-bond

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6
Q

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

A
  • 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
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7
Q

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

A
  • 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
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8
Q

Define:

  • Heterolytic fission (heterolysis)
  • Homolytic fission (homolysis)
A
  • 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)
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9
Q

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

A
  • Bubble gas through bromine water
  • Positive result: red-brown bromine decolourises
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10
Q

What is an addition reaction?

A

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

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11
Q
  • What mechanism occurs when ethene reacts with bromine?
  • Draw the mechanism
  • Describe what occurs during each step
A

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
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12
Q
  • 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?
A
  • ​​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
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13
Q

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.

A
  • 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))
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14
Q

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

A
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15
Q

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

A

Electrophilic addition

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16
Q

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

A

Radical addition (no ions involved)

17
Q
  • What mechanism occurs when alkenes react with water?
  • What is the name of the reaction that occurs?
  • What is produced?
  • What conditions are required?
A
  • Electrophilic addition
  • Hydration (addition of water across double bond)
  • Alcohols
  • H3PO4, steam, heat, pressure OR conc H2SO4 then water
18
Q

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

A
19
Q
  • 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.
A
  • Electrophilic addition

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

20
Q
  • What mechanism occurs when alkenes react with hydrogen?
  • What is the name of the reaction that occurs?
  • What is produced?
  • What conditions are required?
A
  • Electrophilic addition
  • Hydrogenation
  • Alkanes
  • Ni, heat, pressure OR Pt, RTP
21
Q

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
A

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.

22
Q

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
A

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*