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Module 7: Organic Chemistry
Quick questions on Alcohols, oxidation and hydration of alkenes explained: HSC Chemistry Module 7
14short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is structure and the -OH group?Show answer
An alcohol has the general formula $R-OH$ where $R$ is an alkyl group. The functional group is the hydroxyl $-OH$. Alcohols are named using the suffix $-ol$ with a locant for the carbon bearing the OH.
What is classification?Show answer
Classify by counting how many other carbon atoms are bonded to the $C-OH$ carbon.
What is physical properties?Show answer
Boiling point. Hydrogen bonds dominate, so alcohols boil far above alkanes of the same molar mass. Within the alcohol series, boiling point rises with chain length (more dispersion) and falls slightly with branching (less surface contact).
What is oxidation?Show answer
The oxidising agents are acidified potassium dichromate $K_2Cr_2O_7 / H_2SO_4$ (orange to green) or acidified potassium permanganate $KMnO_4 / H_2SO_4$ (purple to colourless). HSC equations use $[O]$ to represent the oxidising agent.
What is production by hydration of alkenes?Show answer
Industrial ethanol is made by adding water across the double bond of ethene, catalysed by dilute sulfuric acid at high temperature and pressure:
What is combustion of alcohols?Show answer
Like hydrocarbons, alcohols burn in excess oxygen to $CO_2$ and water:
What is boiling point?Show answer
Hydrogen bonds dominate, so alcohols boil far above alkanes of the same molar mass. Within the alcohol series, boiling point rises with chain length (more dispersion) and falls slightly with branching (less surface contact).
What is solubility in water?Show answer
Methanol, ethanol, propan-1-ol and propan-2-ol are fully miscible with water. From butanol onwards, solubility drops sharply because the non-polar alkyl tail dominates. By octan-1-ol, the alcohol is essentially insoluble.
What is viscosity?Show answer
Increases with chain length and with the number of OH groups (compare ethanol with ethane-1,2-diol or with glycerol).
What is oxidising a tertiary alcohol?Show answer
It does not happen. If you write a product, you have made an error. The dichromate stays orange.
What is stopping a primary alcohol oxidation at the aldehyde when refluxing?Show answer
Under reflux with excess oxidant, you get the carboxylic acid. The aldehyde is only isolable by distilling it out as it forms.
What is markovnikov direction in hydration?Show answer
Propene plus water gives propan-2-ol (secondary), not propan-1-ol.
What is forgetting the colour change?Show answer
Dichromate: orange to green. Permanganate: purple to colourless. State the change when asked about an observation.
What is confusing fermentation conditions with hydration conditions?Show answer
Fermentation is yeast, 25 to 37 degrees C, anaerobic. Hydration is $H_2SO_4$ catalyst, 300 degrees C, high pressure. Do not mix them.