Alcohols-organic chemistry Flashcards
Production, oxidation, elimination (21 cards)
How are alcohols produced industrially?
Alcohols are produced industrially by hydration of alkenes in the presence of an acid catalyst. This reaction adds water (H₂O) across the double bond of an alkene to form an alcohol.
How is ethanol produced industrially by fermentation?
Ethanol is produced industrially by fermentation of glucose. In this process, yeast converts glucose into ethanol and carbon dioxide in anaerobic conditions. The equation for fermentation is:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
What are the conditions used in the production of ethanol by fermentation of glucose?
The conditions for ethanol production by fermentation are:
Temperature: Around 30-40°C to optimize yeast activity.
Anaerobic conditions (absence of oxygen) to prevent the formation of acetic acid.
Yeast as the catalyst for fermentation.
How is ethanol separated from the fermentation mixture?
Ethanol produced by fermentation is separated from the mixture by fractional distillation. This process separates ethanol based on its boiling point, allowing for the purification of the ethanol.
What is a biofuel?
A biofuel is a fuel derived from biological sources (e.g., plants, algae, or waste) that can be used as an alternative to fossil fuels. Ethanol produced from crops like corn or sugarcane is considered a biofuel.
Is ethanol produced by fermentation a carbon-neutral fuel?
Ethanol produced by fermentation is often considered carbon neutral because the CO₂ released during combustion is absorbed by the plants used in its production, thus balancing out the CO₂ emissions. However, this statement is not entirely valid because:
Energy inputs (e.g., for farming, transportation, and distillation) involve carbon emissions.
Land use changes may impact carbon sequestration, and not all CO₂ emissions are absorbed by plants.
How is ethanol formed by the reaction of an alkene with steam?
Ethanol can be formed by hydration of alkenes using steam in the presence of an acid catalyst (typically phosphoric acid, H₃PO₄). The reaction mechanism involves the following steps:
Electrophilic addition of H⁺ to the alkene.
Nucleophilic attack of water (H₂O) on the carbocation intermediate.
Formation of ethanol.
The equation for this reaction is:
C₂H₄ + H₂O → C₂H₅OH.
What are the environmental and ethical issues associated with biofuels?
Environmental issues include:
Land use for biofuel crops could compete with food production, leading to higher food prices or food shortages.
Deforestation to grow biofuel crops can lead to loss of biodiversity and increased CO₂ emissions.
Ethical issues include:
The impact on food security when crops are diverted for biofuel production instead of for food.
Social and economic equity regarding who benefits from biofuel production (e.g., large corporations vs. small farmers).
Why are specific conditions chosen for ethanol production by fermentation?
The temperature is chosen around 30-40°C because it is the optimal range for yeast activity. If the temperature is too high, the yeast may die, and if it’s too low, fermentation will proceed too slowly. Anaerobic conditions are used to ensure the yeast ferments glucose into ethanol rather than producing acetic acid, which would sour the product.
How are alcohols classified?
Alcohols are classified based on the number of carbon atoms bonded to the hydroxyl group:
Primary alcohols: The carbon bonded to the hydroxyl group is attached to one other carbon atom.
Secondary alcohols: The carbon bonded to the hydroxyl group is attached to two other carbon atoms.
Tertiary alcohols: The carbon bonded to the hydroxyl group is attached to three other carbon atoms.
What happens when primary alcohols are oxidised?
Primary alcohols can be oxidised to form aldehydes. If the oxidation is continued (using an excess of oxidant), the aldehyde can be further oxidised to form a carboxylic acid.
Primary alcohol to aldehyde:
RCH₂OH → [O] → RCHO
Aldehyde to carboxylic acid:
RCHO → [O] → RCOOH
What happens when secondary alcohols are oxidised?
Secondary alcohols can be oxidised to ketones. Secondary alcohols do not undergo further oxidation to carboxylic acids.
Secondary alcohol to ketone:
R₂CHOH → [O] → R₂C=O
What happens when tertiary alcohols are oxidised?
Tertiary alcohols are not easily oxidised because there is no hydrogen attached to the carbon bearing the hydroxyl group, making the oxidation process more difficult.
What is a suitable oxidising agent for alcohols?
Acidified potassium dichromate (VI) is a suitable oxidising agent for alcohols. It is often used to oxidise primary and secondary alcohols, changing from orange to green as it is reduced.
How does the method of oxidation of a primary alcohol determine whether an aldehyde or carboxylic acid is formed?
To obtain an aldehyde, a primary alcohol is oxidised with gentle conditions (e.g., controlled heating).
To obtain a carboxylic acid, the oxidation process uses excess oxidising agent (such as potassium dichromate) under stronger conditions, such as refluxing.
How can aldehydes and ketones be distinguished chemically?
Aldehydes and ketones can be distinguished using the following tests:
Fehling’s solution:
Aldehydes reduce Fehling’s solution (blue → red precipitate).
Ketones do not react with Fehling’s solution.
Tollens’ reagent:
Aldehydes reduce Tollens’ reagent to form a silver mirror.
Ketones do not react with Tollens’ reagent.
How can oxidation reactions of alcohols be represented with [O] as the oxidising agent?
The general oxidation reactions of alcohols can be shown as:
Primary alcohol to aldehyde:
RCH₂OH → [O] → RCHO
Aldehyde to carboxylic acid:
RCHO → [O] → RCOOH
Secondary alcohol to ketone:
R₂CHOH → [O] → R₂C=O
How can alkenes be formed from alcohols?
Alkenes can be formed from alcohols by an acid-catalysed elimination reaction. In this reaction, water (H₂O) is eliminated from the alcohol, resulting in the formation of an alkene.
General reaction:
RCH₂CH₂OH → [H⁺] → RCH=CH₂ + H₂O
What is the mechanism for the elimination of water from alcohols?
The mechanism for the elimination of water from alcohols involves the following steps:
Protonation: The alcohol is protonated by the acid catalyst (often H₃PO₄ or H₂SO₄), which makes the hydroxyl group (–OH) a better leaving group.
Formation of a carbocation: After protonation, the –OH group leaves, forming a carbocation.
Deprotonation: A base (often the conjugate base of the acid catalyst) abstracts a proton from the carbon adjacent to the carbocation, resulting in the formation of an alkene.
What are the uses of alkenes produced by acid-catalysed elimination reactions from alcohols?
Alkenes produced from alcohols can be used to make addition polymers. This is particularly valuable because these alkenes can be derived from renewable resources (such as bioethanol) rather than crude oil, which is the source of many traditional monomers.
Summarize the mechanism of the elimination of water from alcohols.
The mechanism can be summarized as:
The alcohol is protonated by an acid catalyst.
The protonated alcohol undergoes the loss of water, forming a carbocation.
A base abstracts a proton from a neighboring carbon, resulting in the formation of an alkene.
