MC7 - Induction and resonance

What are the  two types of curly arrows? 

1. double headed represent the movement of a pair of electrons

2. single headed (fishhook) represent the movement of one electron ( matters for radicals 

note- arrows only move form a negative ( electron rich ) area to a positive( electron poor) area 

Where can curly arrows move from ?

• a bond

• lone pair

• negative charge

• Never form + all in one direction

Where must the head of the arrow be directed to?

Head must be directed to an electron deficient area:

-positive charge

-positive end of a polarized bond

-an electronegative atom that can accept a pair of electrons

How do reactions start ?

• interactions between electron rich atoms or molecules and electron deficient atoms or molecules 

• can categorise functional groups into being nucleophiles or electrophiles 

What is a nucleophile ?

• An electron rich atom - pair of electrons not involved in bonding which can be shared to form a new bond - alone pair

• An electron rich molecule - a bond with high electron density

Give some examples of an nucleophile

• alcohol

• amines

• water

• alkenes

What is an electrophile ?

An electron deficient atom-

Can accept a pair of electrons to make up a stable octet of electrons

Name two effects that affect how good a nucleophile is ?

• Induction

• Resonance

Nucleophiles can be enhanced or reduced in activity -affected by neighbouring atoms ( the rest of the atom)

These two effects:

-can operate together, or compete with each other

Give a brief description of induction

Induction:

-Occurs through sigma bonds

-Based on electronegativities

-Weak in effect

Give a brief description of resonance ?

Resonance:

-Occurs through pi bonds

-Based on ability to share electron and conjugation

-Strong in effect

-If a group has opposing inductive and resonance effects – RESONANCE WINS ( only if competing - one will withdraw one will donate)

What are the 2 types of inductive affects ?

• I - = electron withdrawing by induction

• I+ = electron donating by induction

what is the effect between carbons and electron rich atoms?

• most common effect - electron withdrawing  by induction 

• can be represented in different ways ( arrows )

When does electron donating by induction occur?

• dipoles can occur between carbon atoms with one being more electron rich 

• carbon is electron donating 

• alkyl groups are electron donating by induction ( I+)

What are the 2 Resonance effects ?

• Resonance Effects (+M, -M)

• Also known as mesomeric effects

• can result in electron withdrawing (-M) or electron donating (+M) effects

Where do these resonance effects result from ?

• from conjugation within a pi system

• The electron density is shared between a number of atoms - delocalised in a single orbital

• how are they represented- RESONANCE FORMS

( extreme examples of the effect )

Describe resonance in benzene

benzene structure: 6 cs, 3 sigma, π bonds ,sp2 hybridized, delocalisation - the π electrons overlap 

How do we represent its delocalisation?

-the bonds rotate bonds are in different places 

-use a resonance arrow to represent they have a resonance relationship 

What happens to the resonance when we add a phenol to benzene?

• phenol is electron donating ( M+) 

• the lone pair of oxygen can feed into the ring and be stabilised by conjugation= larger=better

• 4 resonance forms - actual structure is an average of them all 

resonance forms:

1- normal ,arrow from lone pair to bond the double bond in ring breaks 

2- this results in oxygen gains a + charge - 2 bonds, - ve charge on carbon because it like 4 bonds ,the negative charge then moves round and. the π bond breaks

3- now a - ve charge on that carbon 4 ,and π bonds at c2 ,c5

4-now negative charge moves around to bond the then the π bond breaks and there is now a negative charge on carbon 6 and a double bond on c4 

note - resonance works by forming ,then breaking a bond to form the the bond - goes round in a circle 

What happens when there is competing effects ?

An alcohol group is electron withdrawing by Induction (-I)- weaker

An alcohol group on an aromatic ring is electron donating by resonance (+M)

What happens?- RESONANCE WINS

What happens to resonance when you add a carboxylic acid to benzene

( benzoic acid ) ?

• Benzoic acid- electron withdrawing ( M-)

• carbonyl group is part of extended conjugated system , electrons can be shared  from the ring to the substituent 

• this results in 5 resonance forms 

Resonance forms:

1- the oxygen π bond breaks arrow outwards 

2- results in -ve charge on oxygen and + ve on the carbon , the double bond breaks to squash positive charge

3- there is now a π bond between the ring and chain and a + ve charge on the c2 ,the double bond on carbon 4 breaks to squash + ve charge

4- now a + ve charge on the c4 - and π bonds at c2 ,c5 ,the π bond at c5 - breaks 

5- there is now a π bond on c4 and a + ve charge on. c6

What is the delocalisation /resonance in Carbonyls ?

Delocalisation in Esters and Acids

• the lone pair is shared across the 3 atoms - oxygen ,carbon ,oxygen 

• resonance hybrid - overall -ve electrons are shared across three atoms 

What is the delocalisation of amides/resonance ?

• lone -pair on nitrogen forms double n=bond and oxygen double bond breaks

• + ve charbe on the nitrogen , ,-ve charge on oxygen

What is the effect of induction and resonance ?

• reactivity

• hydrogen bond forming ability

• acidity and basicity