You should recall that the word nucleophile implies a seeker of positive charge.

But what is the exam definition of nucleophile?

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Classify the below as Primary, Secondary or Tertiary haloalkanes

Primary haloalkanes undergo substitution in a two-step mechanism.

The rate equation is:

Rate = k [nucleophile] [haloalkane]

QUESTION: What is the overall order of an S_N2 reaction?

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QUESTION: Insert partial charges (∂+ & ∂-) on the haloalkane below.

QUESTION: Indicate the likely site of attack for a nucleophile

Insert curly arrows to represent the movement of pairs of electrons in the mechanism below

QUESTION: Suggest why the Transition State is unstable

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QUESTION: What is it in the Transition State that indicates partial bonds?

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QUESTION: What do you notice about the stereo geometry of the haloalkane and its product?

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QUESTION: Why might 2ry and 3ry haloalkanes find it harder to react via S_N2

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Tertiary haloalkanes undergo substitution in a one-step mechanism.

The rate equation is:

Rate = k [haloalkane]

QUESTION: What is the overall order of an S_N1 reaction?

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Insert curly arrows to represent the movement of pairs of electrons in the mechanism below. Add + and – charges where needed.

QUESTION: Suggest why the first step is slow

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QUESTION: Suggest why the second step is fast

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QUESTION: Why are 1ry haloalkanes unlikely to react this way?

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S_N1 reactions create carbocations like the one below:

QUESTION: Where does the name carbocation come from?

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Carbocations can be classified as primary, secondary or tertiary

The alkyl inductive effect can push electrons away from alkyl groups

QUESTION: How does the inductive effect explain the relative stability of 1ry, 2ry and 3ry carbocations?

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QUESTION: How does the inductive effect explain why primary haloalkanes follow the S_N2 route but Tertiary haloalaknes follow S_N1 route?

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1. The Halogen

QUESTION: Explain the trend in Bond enthalpy for C-X bonds

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QUESTION: Which type of Haloalkane should react fastest?

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2. Primary, Secondary or Tertiary

It’s harder to compare these directly because:

Primary Haloalkanes almost always follow the S_N___ mechanism

Tertiary Haloalkanes almost always follow the S_N___ mechanism

But Secondary Haloalkanes follow both mechanisms.

So we’re not really comparing like with like.

And conditions affect whether S_N1 or S_N2 is fastest.

However, with S_N1 reactions Tertiary will generally be faster than Secondary due to the stability of the carbocation.

QUESTION: Which forms the most stable carbocation? Tertiary or Secondary?

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QUESTION: Which reacts fastest via S_N1? Tertiary or Secondary?

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Both mechanisms are favoured by polar solvents.

Why is it difficult to dissolve OH^- ions or haloalkanes in non-polar solvents?

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The word Protic in this case means – “able to form Hydrogen Bonds”

What type of covalent bonds must solvents have to Hydrogen bond?

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Why would a protic solvent make OH^- less effective as a nucleophile?

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QUESTION: Complete:

Protic polar solvents favour S_N___ mechanisms because ________

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Aprotic polar solvents favour S_N___ mechanisms because ________

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QUESTION: Why are negatively charged nucleophiles usually better than neutral nucleophiles?

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You should recall that the word electrophile implies a seeker of negative charge.

But what is the exam definition of electrophile?

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Good electrophiles have a + charge, but they could be neutral but with a strong ∂+.

QUESTION: Carbon atoms bonded to which other atoms have strong ∂+ partial charge? Why?

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QUESTION: Electrophiles commonly react with Alkenes. Why?

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Add curly arrows and lone pairs to mechanism below:

QUESTION: C=C bonds contain a σ and a π bond.

Which one breaks during the mecahnism? Why?

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QUESTION: Br₂ is a non-polar molecule

How does it end up with partial charges?

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QUESTION: Cl-Br is a polar molecule

Which halogen atom is ∂+?

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With the mechanism above it doesn’t make any difference which Carbon atom the Hydrogen from the HBr bonds to because Ethene is symmetrical.

But most alkenes aren’t.

You might expect a 50:50 mixture of the two products.

You’d be wrong.

In reality, the secondary haloalkane (top) is the major product, and the primary (bottom) is a minor product.

The stability of the carbocation is the reason.

QUESTION: Which carbocation, top or bottom, is more stable? Why?

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You should already know about Benzene from Topic 10.

QUESTION: Describe the bonding in Benzene.

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QUESTION: Why is the Carbon Carbon bonding in Benzene said to have bond order 1.5?

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QUESTION: Why doesn’t Benzene undergo Electrophilic addition like an Alkene?

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QUESTION: Why does Benzene favour substitution reactions?

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For Electrophilic substitution to happen a positive Electrophile (E+) must be formed.

QUESTION: How does HNO₃ dissociate in water?

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QUESTION: Why wouldn’t NO₃^- be able act as an electrophile?

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Without the Sulphuric acid NO₂⁺ (the Nitronium ion) wouldn’t form.

But the Sulphuric acid reforms at the end.

So the Sulphuric acid is a ______________________.

You should know this reaction requires a 50^oC temperature.

LEARN the two most common reducing agents

LiAlH₄

NaBH₄

QUESTION: What is the oxidation state of the Hydrogen atoms in both of these Reducing Agents?

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What’s the difference?

LiAlH₄ is stronger and so can reduce a Carboxylic acid all the way to ____________________

NaBH₄ is weaker and can only reduce Carbonyls – in other words __________________ & __________________

Equations

Reducing agents are represented by [H], but still need to be balanced.

QUESTION: Complete the reduction of an aldehyde:

CH₃CHO + ___ [H] --> ____________

QUESTION: Complete the reduction of a Carboxylic acid:

CH₃COOH + ___ [H] --> ____________

Neither LiAlH₄ or NaBH₄ are powerful enough to reduce Nitrobenzene.

A mixture of Zinc and Conc HCl is used, followed by NaOH.

QUESTION: Balance the equation for the first step

C₆H₅NO₂ + __ Zn + __ H⁺ à C₆H₅NH₃⁺ + __ Zn²⁺ + __H₂O

QUESTION: What is oxidised in this reaction?

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QUESTION: How do we know the Nitrobenzene was reduced in this reaction?

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QUESTION: Complete the equation for the second step

C₆H₅NH₃⁺ + __ OH^- à C₆H₅NH₂ + _______

QUESTION: What sort of reaction is this?

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