3.5 Alcohols - Properties, Production, and Reactions

Comprehensive review flashcards covering physical properties, industrial production (hydration vs. fermentation), bioethanol carbon neutrality, dehydration/elimination mechanisms, and oxidation reactions of alcohols.

Why do alcohols have a higher boiling point than their corresponding alkanes?

Alcohols can form hydrogen bonds between their molecules, which require significantly more energy to break than the weaker van der Waals forces between alkane molecules.

Why does the solubility of alcohols in water decrease as the length of the hydrocarbon chain increases?

The non-polar hydrocarbon chain only forms van der Waals forces; as the chain lengthens, only a small proportion of the alcohol molecule can form hydrogen bonds with water.

What are three major commercial uses of ethanol listed in the notes?

As a fuel (for example, in E10 petrol containing up to $10\%$ ethanol), as a solvent (such as in hand sanitiser), and as a chemical feedstock to make other chemicals.

What reagents and conditions are required for the industrial hydration of ethene to form ethanol?

Reagents: ethene and steam. Conditions: concentrated $H_3PO_4$ catalyst, a temperature of $300\,^\circ\text{C}$, and a pressure of $6\,\text{MPa}$.

What is the mechanism name for the acid-catalysed hydration of an alkene to an alcohol, and what is the role of concentrated $H_3PO_4$?

The mechanism is electrophilic addition, and concentrated $H_3PO_4$ acts as a catalyst.

What is the overall chemical equation and required conditions for the fermentation of glucose?

Equation: $C_6H_{12}O_6 \rightarrow 2CH_3CH_2OH + 2CO_2$. Conditions: yeast (containing enzymes), anaerobic conditions, and a temperature of $25\text{--}42\,^\circ\text{C}$.

Why must the fermentation of glucose be carried out under anaerobic conditions?

To prevent the oxidation of the ethanol produced.

Why is a compromise temperature of $35\,^\circ\text{C}$ used in the fermentation of glucose?

So that the rate of reaction is fast enough without denaturing the enzymes in yeast.

Why is the percentage yield of ethanol from glucose fermentation limited to approximately $15\%$?

Yeast is killed by concentrations of ethanol above $15\%$.

What separation technique is used to remove ethanol from the fermentation mixture, and what is the purpose of anti-bumping granules?

Fractional distillation is used, and anti-bumping granules are added for smooth boiling. They prevent the formation of large bubbles .

How does the atom economy of ethene hydration compare to that of glucose fermentation?

Hydration of ethene has an atom economy of $100\%$, whereas fermentation has an atom economy of $51.1\%$ ($\frac{2(46.0)}{180.0} \times 100$).

Compare the raw materials, reaction rate, and type of process for ethene hydration versus fermentation.

Hydration uses crude oil (a finite resource), has a fast reaction rate, and is a continuous process. Fermentation uses sugar cane/beet (a renewable resource), has a slow reaction rate, and is a batch process.

What are the definitions of a biofuel and a carbon-neutral process?

A biofuel is a fuel (substance burned to release energy) produced from biological sources. Carbon-neutral describes a process with no net release of $CO_2$ into the atmosphere.

Why is bioethanol theoretically carbon-neutral based on the balanced reaction equations?

Photosynthesis absorbs $6\,\text{moles}$ of $CO_2$ ($-6$), fermentation produces $2\,\text{moles}$ of $CO_2$ ($+2$), and combustion of that ethanol produces $4\,\text{moles}$ of $CO_2$ ($+4$), resulting in a net release of $0\,\text{moles}$ of $CO_2$.

Why might bioethanol not be truly carbon-neutral in practice?

Fossil fuels are burned to power machinery used to fertilise the crops, harvest the crops, and transport the fuel.

What are the disadvantages of using land to grow crops for biofuel production?

The land cannot be used to grow crops for food, fertilisers are often added to the land to increase yield, and deforestation is needed to create land to grow these crops.

What are the three main types of reactions of alcohols?

1) Combustion, 2) Dehydration (elimination mechanism), and 3) Oxidation.

What are the equation, conditions, mechanism name, and role of concentrated $H_2SO_4$ for the dehydration of ethanol?

Equation: $CH_3CH_2OH \rightarrow CH_2=CH_2 + H_2O$. Conditions: concentrated $H_2SO_4$ and heat. Mechanism: elimination. Role of concentrated $H_2SO_4$: catalyst, because $H^+$ is not used up in the process.

In the dehydration of cyclohexanol to cyclohexene, why are saturated $NaCl(aq)$ and $Na_2CO_3(aq)$ added to the product mixture?

Saturated $NaCl(aq)$ increases the density of the aqueous layer to make it easier to remove, ( NaCl is used to remove impurities ) and $Na_2CO_3(aq)$ reacts with and neutralises the acid catalyst.

How is cyclohexene dried after separation, and what test confirms the product is an alkene?

It is dried by the addition of solid $CaCl_2$. The presence of an alkene is confirmed by adding bromine water, which changes from orange to colourless.

What reagents and conditions are used to oxidise a primary alcohol to an aldehyde?

Reagents: acidified potassium dichromate. Conditions: distillation.

Why must an aldehyde be distilled off immediately as it forms during the oxidation of a primary alcohol?

An aldehyde can be oxidised further to a carboxylic acid; because aldehydes have weaker dipole-dipole forces and lower boiling points than alcohols and carboxylic acids, they can be removed first.

What conditions and apparatus are required to oxidise a primary alcohol directly to a carboxylic acid, and how does this method work?

Conditions: acidified potassium dichromate under reflux. Reflux allows heating without loss of volatile compounds because vapours condense and drop back into the flask to be further oxidised.

What reagent and conditions are needed to oxidise a secondary alcohol to a ketone?

Reagents: acidified potassium dichromate. No special conditions are required because ketones cannot be oxidised further.

What observations are seen when aldehydes and ketones are separately tested with Tollens' reagent and Fehling's solution?

Aldehydes produce a silver mirror with Tollens' reagent ($Ag^+ + e^- \rightarrow Ag$) and a brick-red precipitate with Fehling's solution ($Cu^{2+} + e^- \rightarrow Cu^+$). Ketones show no visible change with either reagent.