Alcohols and haloalkanes
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Alcohols are organic compounds which contain the hydroxyl () functional group.
The hydroxyl group contains a oxygen making the adjoining hydrogens and carbon; the overall alcohol molecule is polar.
If in an alcohol is connected to:
- one other carbon, it is a primary alcohol, 1o
- two other carbons, it is secondary alcohol, 2o
- three other carbons, it is tertiary alcohol, 3o
Alcohols exhibit solubility in polar and non-polar solvents. This varies with the length of their carbon chain and the number of groups present.
The group participates in hydrogen bonding and creates molecular polarity. This leads to interaction with polar solvents like water.
The hydrocarbon chain is non-polar and can interact with non-polar solvents.
Short-chain alcohols have good solubility in water and other polar solvents. As the length of the non-polar hydrocarbon chain increases, the solubility of the alcohol in water decreases.
Alcohols exhibit much stronger intermolecular forces than alkanes and alkenes of an equivalent molecular weight.
Alongside the London forces exhibited in both alcohols and similarly sized hydrocarbons, alcohols also exhibit permanent dipole–dipole interactions and hydrogen bonds that are significantly stronger and require more energy to break.
As a result alcohols have higher boiling points and are less volatile than alkanes and alkenes of an equivalent molecular weight.
Alcohols can undergo oxidation in the form of combustion when heated in oxygen, producing water and carbon dioxide.
The complete combustion of butanol is:
Combustion of alcohol is considered cleaner than alkane combustion because it is less likely to produce particulate carbon. This is due to lower oxygen demand for the complete combustion of an alcohol, compared to an alkane of similar molecular weight.
Butanol, 74, burns completely in oxygen with a 1:6 mole ratio, whereas pentane, 72, requires a 1:8 mole ratio.
Alcohols can be oxidised to carbonyl compounds during redox reactions with oxidising agents.
Acidified potassium dichromate VI () is commonly used in the oxidation of alcohols.
Dichromate ions, , turn from orange to green as they are reduced to . The oxidation state of chromium reduces from +6 to +3.
Alcohols can be oxidised to carbonyl compounds during redox reactions with oxidising agents.
Acidified potassium permanganate, (), is a commonly used oxidising agent.
Permanganate ions, -, turn from deep purple to colourless as they are reduced to .
Alcohols react with phosphorus pentachloride () to produce chloroalkanes.
reacts with the alcohol, replacing the with a chlorine atom.
is a strong chlorinating agent suitable for converting primary, secondary, and tertiary alcohols.
The reaction is typically carried out in an inert solvent, such as dichloromethane
Alcohols react with concentrated sulfuric acid and potassium bromide () to form bromoalkanes.
Sulfuric acid reacts with potassium bromide to generate hydrobromic acid, , in situ.
then acts as the nucleophile, substituting the group with a bromine atom.
Alcohols react with red phosphorus and iodine () to produce iodoalkanes.
Red phosphorus reacts with iodine to generate phosphorus triiodide, (), in situ.
facilitates the substitution of the hydroxyl group with an iodine atom.
This reaction is particularly useful for primary alcohols, yielding primary iodoalkanes, which are common intermediate products in organic synthesis.
When oxidised, primary alcohols form aldehydes first before further oxidation to carboxylic acids.
Secondary alcohols are oxidised to form ketones.
![Diagram illustrating the conversion of a secondary alcohol to a ketone. The structure on the left shows a secondary alcohol with an -OH group, while the structure on the right depicts the resulting ketone after oxidation, indicated by the [O] arrow.](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI3MjgwLCJwdXIiOiJibG9iX2lkIn19--160b2431bdd4df098fb59d5a269a6987a947b57d/4_OC_05_Alcohols_011_light.png)
![Diagram illustrating the conversion of a secondary alcohol to a ketone. The structure on the left shows a secondary alcohol with an -OH group, while the structure on the right depicts the resulting ketone after oxidation, indicated by the [O] arrow.](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI3MjgxLCJwdXIiOiJibG9iX2lkIn19--4fa4fbbf92998bdb4a51b60aff92223e6e291c9e/4_OC_05_Alcohols_011_dark.png)
Tertiary alcohols CANNOT be oxidised as they do not have a proton on the carbon bearing the group.
![Diagram illustrating a tertiary alcohol with the hydroxyl group (OH) attached to a carbon atom (C) that is connected to three other groups (R, R', R'). The diagram indicates that when oxidized ([O]), there is no reaction.](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI3MjgyLCJwdXIiOiJibG9iX2lkIn19--7c8fd87ec80b188b474489a31de98be45a90a5a5/4_OC_05_Alcohols_012_light.png)
![Diagram illustrating a tertiary alcohol with the hydroxyl group (OH) attached to a carbon atom (C) that is connected to three other groups (R, R', R'). The diagram indicates that when oxidized ([O]), there is no reaction.](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI3MjgzLCJwdXIiOiJibG9iX2lkIn19--769790c73220b63304e1950550b9e5babd984b76/4_OC_05_Alcohols_012_dark.png)
can be used along the arrow or as a balanced reagent in a chemical equation to represent the oxidising agent.
When oxidising a primary alcohol, aldehydes are isolated through distillation. A carboxylic acid product can be obtained by heating under reflux.
![A diagram illustrating a laboratory setup for the oxidation of a primary alcohol to an aldehyde. The setup includes a round-bottom flask containing a blue liquid labeled 'Primary alcohol and [O]', connected to a condenser with arrows indicating water flow in and out. Below the flask, a heat source is shown, and a receiving flask is positioned to collect the resulting aldehyde.](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI3Mjg0LCJwdXIiOiJibG9iX2lkIn19--fd09ed5d660326129637973c57acb4bc51e79316/4_OC_05_Alcohols_014_light.png)
![A diagram illustrating a laboratory setup for the oxidation of a primary alcohol to an aldehyde. The setup includes a round-bottom flask containing a blue liquid labeled 'Primary alcohol and [O]', connected to a condenser with arrows indicating water flow in and out. Below the flask, a heat source is shown, and a receiving flask is positioned to collect the resulting aldehyde.](/rails/active_storage/blobs/redirect/eyJfcmFpbHMiOnsiZGF0YSI6MTI3Mjg1LCJwdXIiOiJibG9iX2lkIn19--6543d6bf29bd69ead71465fbfa21e6681ea5fe5f/4_OC_05_Alcohols_014_dark.png)
Hydrogen bonding is present in the alcohol but not the aldehyde. As a result, the aldehyde has the lowest boiling point and can be collected by distillation as it forms, preventing further oxidation.
Under reflux, the aldehyde returns to the reaction mixture as it is produced, allowing the secondary oxidation to occur. The carboxylic acid is formed.
The removal of water from an alcohol is an elimination reaction resulting in the formation of an alkene.
Alcohols can also be synthesised from alkenes using an acid catalyst and steam. This process is reversible.
The acid catalyst is or .
or are not used due to the tendency of and to act as competing nucleophiles.
Alkenes produced via acid-catalysed elimination of alcohols serve as valuable monomers for synthesising polymers, offering an alternative to those derived from crude oil.
Alkenes can be derived from biomass, such as plant-based alcohols, reducing reliance on fossil fuels. Compared to fossil fuels, biomass is renewable and has a lower carbon footprint.
The group of an alcohol can be substituted with a halide ion.
The alcohol must be acidified and exposed to a halide nucleophile.
Direct addition of a hydrogen halide (e.g. ) provides both acidity and the halide nucleophile.
The resulting product is a haloalkane.
Alcohols can undergo nucleophilic substitution with the salt of a halide and a strong acid, to form haloalkanes.
Alcohols are acidified with a strong acid, such as or , and then mixed with a salt of the desired halide.
The strong acid provides protons needed to improve quality of the leaving group whilst the salt provides the halide nuclephile. This route is preferred in the formation of bromoalkanes, as is highly corrosive and provides risk of explosion.
is not a suitable acid due to the ability of to act as a competing nucleophile.
Alkenes can be converted to alcohols through an electrophilic addition reaction with steam, in the presence of an acid catalyst.
- Protonation of the alkene
Electrons from the alkene pi bond attack an electrophilic proton. This forms a carbocation intermediate - Nucleophilic attack by water
A water molecule acts as a nucleophile, attacking the positively charged carbocation. - Deprotonation
The oxonium ion loses a proton () to regenerate the acid catalyst and form the final alcohol product.
Alkenes can converted to alcohols on an industrial scale by reacting with steam in the presence of an acid catalyst.
Conditions in industry:
- Catalyst: Concentrated phosphoric acid () on silica
- Temperature: Around
- Pressure: Approximately
Ethanol can be produced industrially by fermenting glucose using yeast under specific conditions. This biological process is essential for producing alcoholic beverages and biofuels.
Glucose () is converted into ethanol () and carbon dioxide ().
Ethanol produced industrially through fermentation is purified using fractional distillation and subsequently utilised.
This process ensures high-purity ethanol suitable for energy applications.
Biofuel refers to a type of fuel that is derived from biological materials (biomass) and is considered renewable and sustainable.
Unlike fossil fuels, which are finite, biofuels are produced from biological sources and can be replenished.
For efficient industrial fermentation of glucose:
- Yeast must be present. This catalyses the conversion of glucose to ethanol.
- An anaerobic environment is required for efficient production of ethanol. Absence of oxygen is crucial to direct the metabolic pathway towards fermentation and ethanol production rather than aerobic respiration, which would produce and reduce the yield of ethanol.
- An optimal temperature range of maintains yeast activity without denaturing the enzymes and maximises fermentation rate.
- An optimal range of 4.0–4.5 ensures yeast enzyme viability and efficient fermentation.
- Adequate nutrients (nitrogen, vitamins, minerals) to support yeast growth and metabolism must be present.
Together these conditions minimise cost and optimises yield.
Ethanol produced by fermentation is often touted as a carbon neutral fuel.
This concept is based on the carbon cycle, where the released during ethanol combustion is offset by the absorbed during the growth of the biomass used to produce ethanol.
Supporting equations for carbon neutrality are:
1. Photosynthesis (carbon fixation)
Plants absorb carbon dioxide from the atmosphere to produce glucose and oxygen.
2. Fermentation (ethanol production)
Yeast converts glucose into ethanol and carbon dioxide.
3. Combustion (ethanol as fuel)
Burning ethanol releases carbon dioxide and water.
Over the duration of the process, the amount of released during fermentation and combustion is equal to the amount consumed by photosynthesis.
The argument for the carbon neutrality of ethanol from fermentation as a fuel is:
“The released during fermentation, and the combustion of ethanol is equivalent to the absorbed during the photosynthesis phase. As long as the biomass (e.g. corn, sugarcane) used to produce ethanol is replanted and the new plants absorb , the overall free level remains balanced.”
There are several factors that invalidate the statement that ethanol produced from biomass is a carbon neutral fuel.
- Energy required for planting, harvesting, processing, transportation, and distillation often comes from fossil fuels, adding to emissions..
- Deforestation may be required to clear land for biofuel crops, reducing the number of trees that can absorb . This results in a net decrease in absorption.
- Production and application of fertilisers consumes energy and causes release of nitrous oxide, another potent greenhouse gas.
- Residual biomass decomposition can release methane, another potent greenhouse gas.
Biofuels are a promising renewable alternative to fossil fuels. When deciding on the appropriateness of biofuels as an energy source, environmental and ethical factors must be considered.
Biofuels can lower emissions compared to fossil fuels, however, production processes may release other more potent, greenhouse gases, such as nitrous oxides and methane.
Large areas of land are required for planting crops for biofuel production. This can lead to deforestation and remove sites for food crop cultivation. Land for biofuel production can also compete with sites to build housing.
High water requirements for growing biofuel crops can strain local water resources. This can be mitigated by sustainable irrigation infrastructure.
Monocultures for biofuels reduce biodiversity and disrupt ecosystems.
Haloalkanes have a polar carbon–halogen () bond.
Due to the higher electronegativity of halogens compared to carbon, electrons in the covalent bond are drawn towards the halogen. This creates a carbon and a halogen.
The polarity of the bond makes the carbon electrophilic and susceptible to nucleophilic attack.
Halogenoalkanes are classified as primary, secondary, or tertiary depending on the number of alkyl groups attached to the carbon.
Primary halogenoalkanes have the general formula .
Secondary halogenoalkanes have the general formula .
Tertiary halogenoalkanes have the general formula .
Tertiary carbocations are the most stable due to the charge-stabilising effect of the three alkyl groups.
The reactivity of haloalkanes follows the stability trend of the carbocations that would be formed following the loss of a halide ion.
Substitution reactions involve the replacement of an atom or group in a molecule by another atom or group.
Haloalkanes commonly undergo substitution reactions, where the halogen atom is replaced by a nucleophile, such as , , or .
Primary haloalkanes react via the generic nucleophilic substitution mechanism shown below.
Partial charges are shown on the bond.
The reaction is initiated by the attack of the 𝛿+ carbon in the haloalkane by an electron pair from the nucleophile. Note that the arrow starts from the lone pair drawn on the nucleophile.
bond formation and bond breaking are simultaneous. The reaction proceeds via a transition state. This is a theoretical state of the molecule that cannot be isolated and represents the maximum in the energy profile.
The final product is inverted, as the nucleophile attacks from behind the bond.
When all other conditions are matched, the trend for the rate of hydrolysis for haloalkanes is as follows:
This directly correlates with the decrease in bond enthalpy of bonds down group , shown in the table.
is also shown to show bond strength in the alkyl group, explaining why fluoroalkanes are highly unreactive.
The conversion of a haloalkane to an alcohol via nucleophilic substitution is called hydrolysis. Halide ions are produced during the reaction.
Ethanolic can be used to identify the type of halogen present, and to study how fast the reaction progresses.
The addition of is a qualitative test in chemistry, as forms different coloured precipitates with halide anions.
Halide ions are formed as the reaction progresses. The reaction rate is taken as the time taken for a silver halide precipitate to become visible.
Generally, the rate of hydrolysis of haloalkane in pure water is slow.
Aqueous alkaline conditions can increase the rate of hydrolysis of a haloalkane. The ion is a more nucleophilic species than water and is present at higher concentration in alkaline conditions.
Water and haloalkanes have low miscibility with each other, but both of them are miscible in ethanol. Adding ethanol to the reaction mixture allows better mixing of reactants, resulting in more collisions per unit time and higher reaction rates.
Nucleophiles can be negatively charged (e.g. ) or neutral with available electron lone pairs (e.g. )
Nucleophiles are electron pair donors.
The term nucleophile comes from the words ‘nucleus’ and ‘philos’ (friend in Greek).
The electrons in a nucleophile are attracted to positively charged areas; areas of low electron density, exposing the positive nuclear charge.
When haloalkanes react with hot ethanolic potassium hydroxide, an elimination reaction occurs. Hydroxide ions are nucleophilic.
In elimination reactions, the hydroxide ion also acts as a base; it abstracts an acidic proton from the -carbon.
Elimination reactions will occure under hot, ethanolic conditions with a strong base like .
In elimination, acts as a base, abstracting a proton () from a -carbon, leading to the formation of a double bond and the elimination of a halide ion.
This contrasts with the role of as a nucleophile in the conditions for nucleophillic substitution, where it directly attacks the carbon.
The double bond formed in elimination reactions can result from the abstraction of a proton from any -carbon. As a result, a mixture of isomers is expected in the product.
It is important to consider the possible positions of the double bond, as well as the E/Z orientation of the groups in the product.
Remember to exclude any products that are superimposable when stating the number of isomers formed.
The ozone layer absorbs and blocks a portion of the ultraviolet (UV) radiation emitted by the sun. This minimises the damage to human skin, as well as to other living organisms, caused by overexposure to this damaging radiation.
Ozone () forms naturally in the upper atmosphere through a series of reactions involving oxygen molecules and UV radiation.
High-energy UV light splits an oxygen molecule () into two oxygen atoms (), which are highly reactive.
Each oxygen atom then reacts with another oxygen molecule to form ozone.
The oxygen atoms formed in the initial splitting of the oxygen molecule are diradicals – they have two unpaired electrons.
Some gaseous haloalkanes, like chlorofluorocarbons (CFCs), can cause damage when allowed to enter the atmosphere. In the upper atmosphere, UV radiation causes homolytic fission of the bond, producing free radicals.
These radicals go on to react with ozone (), reducing its atmospheric abundance.
Fission in a CFC occurs at the bond which is significantly weaker than the bond.
The Montreal Protocol is a treaty signed to reduce the production and release of substances associated with depletion of the ozone layer.
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were developed as propellants and refrigerants but have since been regulated through the Montreal Protocol and subsequent legislation.
Hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) were developed as alternatives to CFCs and HCFCs. HFCs and HFOs do not contain the weak bond linked to the production of chlorine radicals. They are not harmful to the ozone layer but are potent greenhouse gases.
Optimising use of natural refrigerants, such as ammonia, carbon dioxide, and hydrocarbons, is increasingly popular due to their reduced environmental impact.
The overall reaction for the breakdown of ozone is:
The degradation of ozone by radicals in the upper atmosphere is a radical chain reaction.
Initiation: Formation of a halogen radical from the CFC. UV light is required for this step.
Propagation: Reaction between a radical species and a non radical species resulting in a radical and non radical product.
Note that the chlorine radical, which is a reactant in the first propagation step, is a product following the second propagation step, making it catalytic. One radical can destroy many molecules of ozone.
Termination: Reaction between two radical species to form a non radical product.
e.g.