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How do we get from a halogenoalkane to a molecule such as an alcohol, nitrile or amine? These are all examples of nucleophilic substitution reactions.
Nucleophilic substitution reactions are reactions in which a nucleophile attacks a molecule and replaces one of its functional groups.
Let's break the term nucleophilic substitution down a little. First, substitution.
A substitution reaction is a reaction in which one functional group on a molecule is replaced by a different functional group.
Next, let's look at nucleophilic. It refers to nucleophiles.
A nucleophile is an electron pair donor.
Nucleophiles are chemical species that react by donating a lone pair of electrons to an electron-deficient species to form a covalent bond. Nucleophiles are all negatively or partially negatively charged (which we represent using the delta symbol, δ) and feature a lone pair of electrons.
An electron-deficient species is simply a molecule or ion that has an area of positive charge. Any fully or partially positively charged species is electron-deficient.
By looking more closely at the term nucleophile, we can form a picture of what these species actually are. -phile comes from the Greek word philos, which means to love, and nucleo- refers to nuclei, which are the positively charged areas of atoms. Therefore, nucleophiles must ‘love’ positive regions - they are attracted to them.
Examples of nucleophiles include:
Note how ammonia is not an ion. However, it is still a nucleophile, because it has a lone pair of electrons and an atom with a partial negative charge. In this case, that atom is nitrogen.
You'd be forgiven for getting nucleophilic substitution mixed up with a similar term: electrophilic substitution. Whilst the two types of reactions have some features in common, they involve very different species. It is important that you know the difference between them:
Examples of electrophiles include:
However, both nucleophilic substitution and electrophilic substitution are still examples of substitution reactions. This means that they swap one functional group in an organic molecule for a different functional group.
You'll look at an example of an electrophilic substitution reaction in more depth in Reactions of Benzene.
We know that halogenoalkanes are polar molecules (explore Halogenoalkanes to refresh your memory). Because the halogen atom in the C-X bond is a lot more electronegative than the carbon atom, it attracts the shared pair of electrons towards itself. Electrons are negatively charged. This makes the halogen atom partially negatively charged and leaves the carbon partially positively charged.
Nucleophiles, which we now know ‘love’ positive regions, can attack this partially charged carbon atom, in an example of a nucleophilic substitution reaction.
Nucleophilic substitution reactions of halogenoalkanes all follow one of two similar mechanisms. The mechanism used depends on the classification of the halogenoalkane.
As we mentioned, the SN1 and the SN2 reaction mechanisms are similar, but they do have their differences. We explore them both fully in the article Nucleophilic Substitution Mechanism. There, you'll be able to see mechanism diagrams showing electron movement to help you differentiate between the two processes.
You should also note that if your exam board doesn't specifically refer to SN1 or SN2 mechanisms, then when it mentions nucleophilic substitution, it means the SN2 mechanism that is used by primary and secondary halogenoalkanes. And if you don't know the difference between primary, secondary and tertiary halogenoalkanes, check out Halogenoalkanes.
The overall equation for both mechanisms is the same:
RCH2X + Nu- → RCH2Nu + X-
We've shown the equation using a primary halogenoalkane for simplicity, but it is easy enough to adapt it to fit other halogenoalkane classifications. You merely need to swap one or both of the halogenoalkane's hydrogen atoms with an extra one or two R groups.
The halogen, or halide ion, is known as the leaving group.
A leaving group is a fragment of a molecule that leaves the parent molecule in a chemical reaction. When the chemical bond joining the leaving group and parent molecule together is broken, the bonding pair of electrons moves over to the leaving group.
Some halogens are more capable to act as a leaving group than others. This means that they react much more readily in nucleophilic substitution reactions. Their ability to act in this way increases as you move down the periodic table and is thanks to atomic radius.
For example, fluoroalkanes with C-F bonds do not undergo nucleophilic substitution whereas iodoalkanes with weak C-I bonds react rapidly with nucleophiles. This is because iodine is a much larger atom than fluorine. Its valence electrons are a lot further away from its nucleus and the C-I bond is longer than the C-F bond. This means the bond has a much lower enthalpy and requires less energy to break. The bond is more reactive. Bond lengths increase as you move down the group in the periodic table. Therefore, the reactivity of halogenoalkanes in nucleophilic substitution increases as you move down the group.
For more information on halogenoalkanes and their reactivity, see Halogenoalkanes.
Above, we saw how nucleophilic substitution can have an SN1 or SN2 mechanism. These two different mechanisms produce products with different stereochemical aspects:
We've shown these stereochemical aspects using a halogenoalkane nucleophilic substitution reaction:
Notice how in the SN2 mechanism above, the bonds in the product are inverted compared to the original reacting molecule. Compare this to the SN1 mechanism, which produces two different enantiomer products. One of the products is inverted, whilst the other keeps the original arrangement of bonds.
Once again, stereochemical aspects of nucleophilic substitution will all become clearer in Nucleophilic Substitution Mechanism. You can also learn more about stereoisomers and racemic mixtures in Optical Isomerism.
Let's now move on to examples of nucleophilic substitution. We'll focus on nucleophilic substitution reactions involving halogenoalkanes.
Halogenoalkanes can react with the hydroxide ion, cyanide ion and ammonia molecule in nucleophilic substitution reactions. These reactions all use either the SN2 or SN1 mechanism that we looked at earlier, depending on the classification of the reacting halogenoalkane. Remember:
Halogenoalkanes react with aqueous sodium or potassium hydroxide (NaOH or KOH) to form an alcohol (ROH) and a halide ion (X-). Alcohols have the hydroxyl functional group (-OH) and are represented by the general formula CxH2x+1OH. The potassium/sodium ion acts as a spectator ion and is not shown in the mechanism.
A spectator ion is an ion that remains in the same form on both sides of the reaction equation. It keeps the same physical state, charge, and oxidation state.
If we write out all the ions involved in a reaction, we can see which are spectators. For example, in the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) , the sodium ions (Na+) and chloride ions (Cl-) are all spectators - they stay in the same state and aren’t changed in the reaction.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
+ Cl-(aq ) + Na+(aq) + OH-(aq) → Na +(aq) + Cl -(aq) + H2O(l)
Let’s look at the conditions for nucleophilic substitution with hydroxide ions. Halogenoalkanes do not readily mix with water, so ethanol is used as a solvent for the substitution reaction. The mixture is warmed under reflux to increase the rate of reaction:
For more on rates of reaction, see Collision Theory and Increasing Rates.
For example, bromoethane (CH3CH2Br) reacts with potassium hydroxide to form ethanol (CH3CH2OH) and a bromide ion. The bromide ion then reacts with the potassium ion to form potassium bromide. This can be shown by the following overall equation:
CH3CH2Br + KOH → CH3CH2OH + KBr
Remember to use structural formulae when writing equations to show the molecule’s structure and the position of the new functional group.
Another example is the nucleophilic attack of 2-chloro-2-methylpropane (CH3CCl(CH3)CH3) by sodium hydroxide, forming 2-methylpropan-2-ol (CH3COH(CH3)CH3) and sodium chloride. Here's the equation:
CH3CCl(CH3)CH3 + NaOH → CH3CCl(CH3)CH3 + NaCl
There is another type of reaction involving halogenoalkanes and hydroxide ions. It is called an elimination reaction. In elimination reactions, the hydroxide ion acts as a base instead of a nucleophile. It produces an alkene, water and a halide ion. The conditions are slightly different too - we use hot and concentrated ethanolic potassium (or sodium) hydroxide. Explore this in Elimination Reactions.
Potassium or sodium cyanide (KCN or NaCN) react with halogenoalkanes in ethanolic solution to form a nitrile (RCN) and a halide ion. Nitriles have the functional group -CN, which contains a C≡N triple bond. Once again, the reaction mixture is heated under reflux. This nucleophilic substitution reaction is important industrially as it increases the length of the carbon chain.
For example, chloromethane (CH3Cl) heated in ethanolic potassium cyanide produces ethanenitrile (CH3CN) and a chloride ion. The chloride ion then reacts with potassium to form potassium chloride. The overall equation is as follows:
CH3Cl + KCN → CH3CN + KCl
For more information on nitriles, see the article specifically dedicated to them: Nitriles.
The reaction between halogenoalkanes and an excess of ammonia (NH3) produces a primary amine (RNH2), a halide ion and an ammonium ion (NH4+). Amines are ammonia derivatives, where one or more of the hydrogen atoms has been replaced by an alkyl group.
We saw earlier in the article that although ammonia is not a negative ion, it is still a nucleophile. It contains a partially negatively charged atom, Nδ-, with a lone pair of electrons. When the nitrogen atom donates its lone pair of electrons to the carbon atom, the nitrogen atom becomes positively charged. This isn’t great for the molecule - it wants to be neutral, as that’s a lot more stable. To solve this problem, it kicks out a hydrogen atom, but keeps the bonded pair of electrons. The hydrogen atom is now a positive ion, and reacts with a second molecule of ammonia to form a positive ammonium ion. This positive ammonium ion can then react with the bromide ion produced in the substitution reaction, forming an ammonium salt. Overall, the reaction requires two moles of ammonia for each mole of halogenoalkane.
The reaction is carried out heated in ethanolic solution, in a sealed container under pressure.
For example, bromoethane (CH3CH2Br) and ammonia react together to form ethanamine (CH3CH2NH2), a bromide ion, and an ammonium ion. The ammonium ion reacts with the bromide ion to form an ammonium salt, ammonium bromide (NH4Br):
CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br
Let's now consider the reaction of halogenoalkanes with silver nitrate solution (AgNO3(aq)) mixed with ethanol. We use this process to identify the halogen present in the halogenoalkane. The ethanol acts as a solvent and allows everything to dissolve, whilst the water in the silver nitrate solution acts as the nucleophile, producing an alcohol, a hydrogen ion (H+), and a halide ion (X-). The halide ion then reacts with the silver nitrate to form a coloured precipitate, and the colour of the precipitate gives us the identity of the halogen.
|Halogen present||Reaction with AgNO3(aq)||Further details|
|Fluorine||No observable reaction||No precipitate as AgF is soluble in water|
Because this reaction produces a visible precipitate, it is a great way to compare the relative rates of reaction of different halogenoalkanes:
Finally, we'll discuss the importance of nucleophilic substitution reactions.
A nucleophilic substitution reaction is a reaction in which a functional group on a molecule is replaced by a nucleophile. Nucleophiles are electron pair donors with a negative or partial negative charge and a spare pair of electrons.
Benzene derivatives, such as chlorobenzene, can undergo nucleophilic substitution reactions. However, benzene itself undergoes electrophilic substitution instead. This is because the high charge density of its ring of delocalised electrons is appealing to electrophiles.
There are multiple types of nucleophilic substitution reactions. Examples include the substitution of halogenoalkanes using hydroxide ions or cyanide ions.
Factors affecting nucleophilic substitution reactions include the partial charge of the carbon atom, the strength of the bond between the carbon and the leaving group, and the strength of the nucleophile.
What is a nucleophile?
An electron pair donor with a negative or δ- charge, and a lone pair of electrons.
The main product of the reaction between a hydroxide ion and a halogenoalkane is:
Give the conditions for the nucleophilic substitution reaction between cyanide ions and a halogenoalkane.
Warm, reflux, ethanolic.
Give the conditions for the nucleophilic substitution reaction between hydroxide ions and a halogenoalkane.
Aqueous and ethanolic, heat, reflux.
Why is reflux used in the nucleophilic substitution of halogenoalkanes with sodium hydroxide?
It prevents the escape of volatile components before they can react and increases the time available for reaction.
What is a substitution reaction?
A reaction in which one functional group is removed from a molecule and replaced by a different functional group.
Which type of halogenoalkane reacts the fastest in nucleophilic substitution reactions?
Which type of halogenoalkane reacts the fastest in nucleophilic substitution reactions?
What sort of mechanism is used in the nucleophilic substitution of primary halogenoalkanes?
What sort of mechanism is used in the nucleophilic substitution of secondary halogenoalkanes?
Both SN1 and SN2
What sort of mechanism is used in the nucleophilic substitution of tertiary halogenoalkanes?
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