Module 4 - [ch14,15] (alcohols, haloalkanes) Flashcards by Najdah G

why is methanol useful

efficient combustion - used as a high performance fuel
starting material in many industrial syntheses
(e.g to produce paint, adhesives ect.)

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physical properties of alcohols in comparison to alkanes

higher melting points
more soluble in water

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why are alcohols more soluble than alkanes in water

can form hydrogen bonds with water molecules

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why do alcohols have a higher boiling points than alkanes

alcohol has hydrogen bonds
stronger than london forces in alkanes
require more energy to overcome
less volatile than alkanes

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describe hydrogen bonding in alcohols

alcohols have a polar O-H bond due o difference in electronegativity of the oxygen and hydrogen atoms
therefore polar
-OH groups form strong hydrogen bonds

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how does solubility of alcohols differ to that of alkanes as chain length increases

as hydrocarbon chain length increases, the influence of the -OH decreases
solubility of longer-chain alcohols becomes to similar to hydrocarbons

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how are alcohols classified

primary, secondary and tertiary
based on number of hydrogen atoms and alkyl groups attached to the carbon atom with the -OH group

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what are the two primary alcohols

methanol, ethanol

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what do you call an alcohol where the carbon with the -OH group is bonded to no hydrogen atoms and 3 alkyl groups

tertiary

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combustion of alcohol

alcohol + oxygen -> carbon dioxide + water

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oxidation of alcohols

alcohol + oxidising agent -> carboxylic acid

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which alcohols can be oxidised

primary and secondary

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the example of oxidation you need to know

potassium dichromate(VI)
K2Cr2O7
acidified with dilute sulfuric acid
H2SO4
oxidised - orange -> green
orange ( dichromate(VI) ions)
green (chromium(III) ions)

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what are primary alcohols oxidised to

aldehydes or carboxylic acids

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how do you prepare an aldehyde

gentle heating of primary alcohol with acidified potassium dichromate

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why is aldehyde is distilled out of the reaction mixture as it forms

to prevent further reaction with the oxidising agent
aldehydes can be oxidised to carboxylic acids

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how do you prepare carboxylic acids

heat primary alcohol strongly under reflux with an excess of acidified potassium dichromate(VI)

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why is an excess of acidified potassium dichromate (VI) used in preparation of a carboxylic acid

ensures all of the alcohol is oxidised

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why do you heat under reflux when preparing a carboxylic acid from a primary alcohol

ensures any aldehyde formed initially in the reaction also undergoes oxidation to become a carboxylic acid

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oxidation of secondary alcohols

ketones

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what is done to ensure the oxidation of secondary alcohols goes to completion

the secondary alcohol is heated under reflux with the oxidising mixture
dichromate, orange-.green

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what is done to ensure the oxidation of secondary alcohols goes to completion

the secondary alcohol is heated under reflux with the oxidising mixture
dichromate, orange-.green

oxidation of tertiary alcohols

do not undergo oxidation reactions
acidified dichromate remains orange

describe dehydration of alcohols

alcohol heated under reflux
in the presence of an acid catalyst (H2SO4, H3PO4)
to form an alkene + water

what type of reaction is the dehydration of an alcohol

elimination

alcohol + hydrogen halide

haloalkanes

how do you prepare a haloalkane

alcohol heated under reflux with sulfuric acid and a sodium halide substitution reaction

are haloalkanes reactive and why

yes carbon - halogen bond halogen more electronegative than carbon electron pair closer to halogen atom polar bond carbon has a delta positive charge acts as a electrophile and attracts a nucleophile

nucleophilic substitution

when a haloalkane reacts with a nucleophile nucleophile replaces the halogen in a substitution reaction

three most common nucleophiles

hydroxide ions water molecules ammonia molecules

nucleophile

lone electron pair donor

what is hydrolysis

chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule

hydrolysis of a haloalkane

halogen atom is replaced by an -OH group nucleophilic substitution

describe the mechanism for hydrolysis of a haloalkane

nucleophile OH~ approaches the carbon attached to the halogen from the other side direction of attack minimises repulsion between negative charges bond formed between carbon atom and oxygen of hydroxide ion C - halogen bond broken by heterolytic fission alcohol and halide ion formed

how can haloalkanes be converted to alcohols (not hydrolysis)

aqueous sodium hydroxide (NaOH) slow reaction at room temp mixture heated under reflux to obtain good yield

what does the rate of hydrolysis depend on

strength of the carbon-halogen bond

how do the carbon-halogen bonds vary

C-F is the strongest largest difference in electronegativity unreactive as much more energy required to break this bond rather than C-I

how would you measure the rate of hydrolysis

haloalkane + water carry out these reactions in presence of aqueous silver nitrate Ag+ ions react with the X~ ions as they are formed to form precipitate AgX AgCl - white AgBr - cream AgI - yellow

why is ethanol solvent used rather than water in the hydrolysis of primary haloalkanes

ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers haloalkanes are insoluble in water

the rate of hydrolysis increase as...

carbon-halogen strength decreases

what are organohalogen compounds

molecules that contain at least one halogen atom joined to the carbon chain

uses of organohalogen compounds

pesticides making polymers flame retardants dry cleaning solvents

what are CFCs

chlorine-fluoro-carbons haloalkanes containing only Cl,F,C

what are the issues regarding CFCs

catalyse the breakdown of the ozone layer via free radical substitution

what is the main function of the ozone layer

provides protection from harmful UV radiation

does the ozone play a protective role everywhere

contributes to photochemical smog in the trotosphere

what does ozone decompose to write an equation for this

oxygen 2O3 -> 3O2

where is the ozone layer

outer edge of the stratosphere height from 10-40km above earth's surface

how is ozone formed

O2 broken into oxygen radicals initially high energy UV breaks oxygen molecules O2 + O -> O3 steady state set up involving formation of ozone at same rate of breaking down of the ozone use of CFCs and other human activity has upset this delicate equilibrium

impact of CFCs on the earth's atmosphere

CFCs remain stable until they reach the stratosphere begin to break down to form chlorine radicals which catalyse the breakdown of the ozone layer

what is photodissociation

when radiation initiates the breakdown

free radical substitution reactions to show how cl catalyses the breakdown of O3

Cl2 -> 2Cl . Cl . + O3 -> ClO. + O2 ClO. + O3 - > 2O2 + Cl . overall equation 2O3 - > 3O2

free radical substitution equations to show how nitrogen monoxide can decompose the ozone

.NO + O3 -> NO2. + O2 NO2. + O -> NO. + O2 O3 + O -> 2O2

how are nitrogen oxide radicals formed

naturally during lightning strikes as a result of aircraft flying in the stratosphere