chapter 4- alcohols

general formula of alcohols?

• CnH2n+1OH

what are the 3 types of alcohols?

• primary, secondary, tertiary

  • this is dependent on which carbon atom the -OH is bonded to

what prefix is used when the compound has an -OH group, in addition to another functional group?

• -hydroxy

why are alcohols polar?

• because of the electronegative hydroxyl group which pulls the electrons in the C-OH bond away from the carbon atom

what type of bonding do alcohols contain?

• hydrogen bonding

  • the strongest kind of intermolecular force

• this is why alcohols have low volatility

in what reaction do alcohols react with compounds containing halide ions?

• substitution reaction

  • the hydroxyl group is replaced by the halide, so the alcohol is transformed to form a haloalkane

  • this reaction also requires the addition of an acid, such as sulfuric acid

• water is a by-product of the reaction

what is the reaction where alkenes are made from alcohols?

• elimination reaction

  • the alcohol is mixed with an acid catalyst (either concentrated sulfuric acid or concentrated phosphoric acid). the mixture is then heated

  • when an alcohol dehydrates, it eliminates water

• the water molecule is made up from the hydroxyl group and a hydrogen atom that was bonded to a carbon atom adjacent to the hydroxyl carbon

• this means that there are often 2 possible alkene products from one elimination reaction depending on which side of the hydroxyl group the hydrogen is eliminated from

  • alkene products could also form E/Z isomers

mechanism of elimination?

mechanism of elimination for primary alcohols?

why do we need a catalyst for elimination reaction?

• the catalyst protonates the -OH group to make it easier to eliminate the group

• the acid catalyst is reformed when the H+ is lost as the alkene forms in the last step

when alcohols burn in oxygen, what do they produce?

• carbon dioxide and water

what happens when alcohols are burned in insufficient oxygen?

• then we get incomplete combustion

• depending on how much oxygen is present, we could get a mixture of carbon monoxide, carbon (soot) and water

what is a primary alcohol?

• an alcohol where the carbon is bonded to the functional group

  • e.g- ethanol

what is a secondary alcohol?

• an alcohol where the carbon bonded to the functional group, is bonded to 2 other carbons

  • e.g- propan-2-ol

what is a tertiary alcohol?

• an alcohol where the carbon bonded to the functional group is bonded to 3 other carbons

  • e.g- 2-methyl-propan-2-ol

what happens when primary alcohols are oxidised?

• they are oxidised to aldehydes, then carboxylic acids

what happens when secondary alcohols are oxidised?

• they are oxidised to ketones only

what happens when tertiary alcohols are oxidised?

• they won’t be oxidised

functional group of aldehydes and ketones?

• CnH2nO

what is the oxidising agent needed for oxidising alcohols?

• acidified dichromate

  • e.g- K2Cr2O7/H2SO4

how can you control how far an alcohol (primary) is oxidised by controlling the reaction conditions?

• distil for an aldehyde

• reflux for carboxylic acid

how to produce an aldehyde?

• carry out the reaction under distillation

  • this condenses any reacted aldehyde, which forms as a gas, into a new flask

how to produce a carboxylic acid?

• carry out the reaction under reflux

  • this involves the use of a reflux condenser

  • the reflux condenser condenses any gases and forces them back into the reaction vessel

  • this puts the aldehyde back in the flask to be oxidised again

what is the only way to oxidise tertiary alcohols?

• by burning them

tollens reagent?

• ALDEHYDES only are oxidised by tollens reagent into a carboxylic acid

• the silver ions are reduced to silver atoms

  • with aldehydes, a silver mirror forms coating inside the test tube

  • with ketones, there is no visible change

fehling’s solution?

• ALDEHYDES ONLY are oxidised by fehling’s solution into a carboxylic acid.

• the copper (II) ions are reduced to copper (I) oxide

  • in aldehydes, the blue CU2+ ions in the solution change to a red precipitate of CU2O

  • ketones do not react

how can the presence of a carboxylic acid be tested?

• by the addition of sodium carbonate

  • it will fizz and produce carbon dioxide

what 3 things does reflux do?

• thermally accelerate the reaction by conducting it at a constant elevated temperature

• keeps volatile substances in the reaction vessels

• keeps volatile flammable solvents away from the heat source

reflux apparatus set up and purification?

• apparatus:

  • round bottom flask

  • heat source

  • condenser

• purification:

  • separating funnel to separate organic and aqueous layer

  • drying with anhydrous salt, like magnesium sulfate

  • redistillation, collect fraction distilling at 102C

distillation conditions- forming aldehyde and carboxylic acid?

• forming aldehyde:

  • h2so4/k2cr2o7

  • heat AND distillation

  • organic product is …..

  • equation is ….

• forming carboxylic acid

  • h2so4/k2cr2o7

  • heat under reflux

  • organic product is ….

  • equation is ….

• distillation set up:

  • diagram of apparatus with condenser

  • condenser had water flow

  • collection of organic product

  • product separated to prevent further oxidation

reagents and conditions for oxidation of primary alcohol to aldehyde?

• reagents: potassium dichromate (VI) solution and dilute sulfuric acid

• conditions: use a LIMITED amount of dichromate, warm gently and use distillation

reagents and conditions for oxidation of aldehyde to carboxylic acid?

• reagents: potassium dichromate (VI) solution and dilute sulfuric acid

• conditions: use an EXCESS of dichromate and heat under reflux

why is an aldehyde likely to be produced with distillation apparatus?

• volatile compounds can escape/distil out

why is a carboxylic acid most likely to be produced with reflux apparatus?

• volatile compounds can not escape/distil out

• complete oxidation will be achieved/oxidised to the acid

equation for formation of bromoethane from ethanol?

(haloalkane to alcohol)

• NaBr + H2SO4 —→ NaHSO4 + HBr

• CH3CH2OH + HBr ——→ C2H5Br + H2O

why do alcohols have relatively low volatilities compared to alkanes?

• due to their ability to form hydrogen bonds between alcohol molecules

solubility of alcohols in water compared to alkanes?

• the smaller alcohols (up to 3 carbon) are soluble in water because they can form hydrogen bonds with water

• the longer the hydrocarbon chain, the less soluble the alcohol