Halogenoalkanes are more reactive than alkanes.
Reveiw – Why are alkanes unreactive? If you can’t answer this question, you need to review alkanes.
✍️ Draw and name all of the isomers of C4H9Br and classify them as primary, secondary and tertiary.
What is different about halogenoalkanes that makes them more reactive than alkanes? Consider the two points that make alkanes less reactive. Is there any differences with halogenoalkanes?
Nucleophilic Substitution Reactions of Halogenoalkanes
✍️ Define a nucleophile and give 3 different examples.
We are only going to be concerned with using hydroxide ion (OH–) in aqueous solution as our nucleophile.
✍️ (HL) Why is hydroxide ion a better nucleophile than water? Check and see if you agree with the video below:
When a halogenoalkane is reacted with aqueous OH– an alcohol is produced.
✍️ Draw and name the products of nucleophilic substitution with all the isomers of C4H9Br you drew earlier.
We have finished with the standard level material in this section. Time for review!
Nucleophilic Substitution Mechanisms (HL only)
Depending on whether the halogenoalkane is primary, secondary or tertiary, depends on the mechanism for this reaction.
Primary halogenoalkanes tend to react via a SN2 mechanism.
Tertiary halogenoalkanes tend to react via a SN1 mechanism.
Secondary halogenoalkanes use either and you can’t predict which one.
Examine the two mechanisms. They are written for any halogenoalkane and any nucleophile.
Key to the mechanisms:
L = leaving group. This is the halogen F, Cl, Br or I.
Nu– = nucleophile. This could be OH– or any other species with a lone pair.
Now have a look at the following animation. Here is the link if you want to see the original.
There is more than one type of mechanism here so choose unimolecular nucleophilic substitution for SN1 and bimolecular nucleophilic substituion for SN2.
✍️ After examining the mechanisms and the animation, try and answer the following questions:
- What does S and N stand for in the notation of the mechanism (SN2)?
- The numbers 1 and 2 stand for the molecularity of the mechanism. What does this mean?
- Define the terms unimolecular and bimolecular.
- Which mechanism has a carbocation intermediate? Identify it.
- Which mechanism forms a transition state?
- What are the coloured arrows trying to indicate in the SN1 mechanism?
Conditions for the Reactions
SN1 reactions are best conducted using protic, polar solvents.
SN2 reactions are best conducted using aprotic, polar solvents.
Polar, aprotic solvents include:
Polar, protic solvents include:
- propan-1-ol and propan-2-ol
- ethanoic acid
✍️ Draw the structures for the two groups of solvents.
✍️ What is the difference between an aprotic and a protic solvent?
Rate of Reaction
Examine the two mechanisms again.
✍️ Write a rate equation for each mechanism.
How does the type of halogen affect the rate of reaction?
✍️ Using your data booklet fill in the electronegativities and bond enthalpies for the difference carbon-halogen bonds.
C-X Electronegativity Bond enthalpy
✍️ What trends to you notice? Discuss these with your table.
✍️ Which halogenoalkane would a nucleophile be most attracted to?
Despite the polarity of the bonds, the most important factor in determining rate is bond strength.
✍️ Knowing this, rank the halogenoalkaness in order from fastest to slowest for reaction with a nucleophile.
Everything you need to know about these two mechanisms is summarised on this sheet here.
✍️ Before trying the review questions, read the relevant section in your text and annotate your notes with any extra important information.