What is nucleophilic substitution and how can I draw a mechanism to show this reaction taking place?

1) A substitution reaction is a displacement reaction. This is when one functional group in one reactant is displaced by another functional group from a second reactant.

This is different to an addition reaction where two reactants add together to form one product. 

2) A nucleophilic substitution means a nucleophile is the attacking species. 

Nucleophiles are 'nucleus loving' species. Nuclei are positively charged. Opposites attract, therefore nucleophiles will have negative or slightly negative charges.

A nucleophile is an electron pair donor. It will use a lone pair of electrons to form a bond with another atom. This will cause displacement of the functional group originally attached. This becomes known as the leaving group.

To draw mechanisms we just need to learn a few simple rules, example -OH and halogenoalkanes:

1) Identify the nucleophile and the electophile e.g. -OH is a nucleophile. It has a lone pair of electrons localised on the oxygen. This is shown by the negative charge. The slightly positive carbon attached to a halogen in a halogenalkane can act as an electophile. This carbon is slightly positive due to the electron withdrawing effect of halogens.

2) Draw a double headed curly arrow from the lone pair on the nucleophile (O-) to the electophile (C attached to halogen). This represents a new bond being made.

3) When a new bond is made an old bond must break. Carbon can only form 4 bonds.

4) Draw a second curly headed arrow from the middle of the carbon-halogen bond to the halogen atom. This represents the halogen group leaving as a leaving group, taking with it the pair of electrons from the bond. It therefore forms a halide ion e.g. Cl-

5) Draw out your new products - an alcohol and halide ion.

6) TIP - count your carbons! Don't loose any carbon atoms on the way, this often happens when using skeletal formulae.

Amber B. IB Chemistry tutor, GCSE Chemistry tutor, A Level Chemistry ...

2 months ago

Answered by Amber, an A Level Chemistry tutor with MyTutor

Still stuck? Get one-to-one help from a personally interviewed subject specialist


Hibba S. A Level Biology tutor, GCSE Biology tutor, GCSE Chemistry tu...
View profile
£20 /hr

Hibba S.

Degree: BSc Biomedical Science (Bachelors) - Queen Mary, London University

Subjects offered: Chemistry, Science+ 3 more

Business Studies

“I am a BSc Biomedical student at Queen Mary University London. I enjoyed learning both Economics and Sciences in A-levels, which I hope I can inspire in you to! From GCSE’s to A levels I have been involved in many extra-curricular ac...”

Ben A. Mentoring -Medical School Preparation- tutor, Uni Admissions T...
View profile
£20 /hr

Ben A.

Degree: Medicine (Bachelors) - University College London University

Subjects offered: Chemistry, Physics+ 6 more

.BMAT (BioMedical Admissions)
-Personal Statements-
-Medical School Preparation-

“Hello,I am a first year medical student at University College London, When I was in sixth form, I helped tutor the younger years and I always enjoyed it, so I decided to keep it on in my university years.My subjects are the three m...”

PremiumGeorgina U. A Level Chemistry tutor, GCSE Chemistry tutor, GCSE Biolo...
View profile
£24 /hr

Georgina U.

Degree: Chemistry (Masters) - Durham University

Subjects offered: Chemistry, Science+ 4 more

Human Biology
-Personal Statements-

“Hi, I'm Georgie! :) A Chemistry graduate/enthusiast who's looking to tutor and support science learning from 13+ right through to A-level. I'd also love to help you with university applications (including ones for medical school). ”

About the author

Amber B. IB Chemistry tutor, GCSE Chemistry tutor, A Level Chemistry ...
View profile

Amber B.

Currently unavailable: for new students

Degree: Pharmacy (Masters) - Kings, London University

Subjects offered: Chemistry, Extended Project Qualification+ 2 more

Extended Project Qualification
-Personal Statements-

“About me: I am a second year Pharmacy student at Kings College London with a love for scienceI hope I can pass onto you through our sessions.  I am very understanding and easy to talk to and have previous tutoring and teaching experi...”

You may also like...

Other A Level Chemistry questions

What is the difference between Sn1 and Sn2 reactions?

Dehydration reaction involving ethanol will produce ....

Define the terms atomic number and mass number

What is Le Chatelier's principle?

View A Level Chemistry tutors


We use cookies to improve our service. By continuing to use this website, we'll assume that you're OK with this. Dismiss