Organics - mechanisms - conditions etc.

what is the initiation step for free radical substitution

formation of free radicals

what are the two propagation steps for free radical substitution

formation of products and regeneraion of original free radical

why is free radical substitution a chain reaction

the original free radical is regenerated

what is the termination step for free radical substitution

combination of two free radicals

what must be present for an alkane and a halogen to react

UV light

what would you observe when you shine UV light onto a mixture of an alkane and bromine water

the mixture becomes colourless and misty fumes of hydrogen bromide appear

why do 2 halogen free radicals form when the mixture is under UV light?

the chlorine molecules absorbs the energy of a single quantum of UV light. the energy of one quantum of UV light is greater than Cl-Cl bond energy, so the bond will break.

why is the formation of two chlorine free radicals described as homolytic?

both atoms gain one electron

why does the Cl-Cl bond break homolytically

both atoms are the same

free radicals are highly what

reactive

how do you test for hydrogen halides

waft the fumes from a bottle of ammonia over the mouth of the test tube. white fumes of ammonium halides are formed.

why does the C-H bond in the alkane not break in the initiation stage of free radical substitution?

C-H bond in the alkane requires more energy to break than is available in a quantum of UV radiation

why are chain reactions not very useful?

they produce a high mixture of products

when may free radical substitution occur without UV light?

at high temperatures

what are CFCs

chlorofluorocarbons

what is it believed that CFCs do

destroying the ozone layer by decomposing ozone (O3) to oxygen

write the equation for the decomposition of ozone to oxygen (use asterix to show free radicals)

Cl* + O3 → ClO* + O2

ClO* +O3 → 2O2 + Cl*

what does the chlorine free radical act as in the breakdown from ozone to oxygen?

a catalyst

what is a nucleophile

a negatively charged ion or has an atom with a delta negative charge

what do nucleophiles do

attack and form bonds with positively or partially positively charged carbon atoms

what must nucleophiles have

a lone pair of electrons situated on an electronegative atom which it can use to form a covalent bond

what affects the rate of reaction for nucleophilic substitution of halogenoalkanes?

the strength of the C-X bond: the longer the bond, the weaker the bond, the more easily it breaks, and the faster the reaction

what is needed to form a primary amine in a nucleophilic substitution reaction with NH3 and a halogenoalkane

an excess of NH3

what does the OH- ion act as in an elimination reaction between a halogenoalkane and hot, ethanolic KOH

a base

what are electrophiles

lone pair acceptors

why does a double bond in an alkene have restricted rotation

there is a single C-C bond and there is a p-orbital containing a single electron on each carbon. the orbitals overlap which form an orbital with a cloud of electron density above and below the single bond. this is called a pi orital and its presence restricts rotation.

what increases how stable a carbocation is?

the more alkyl groups there are attached to the positively charged carbon atom

Why does the stability of carbocations increase with the number of alkyl groups attached to the positively charged carbon atom?

the positive inductive effect. alkyl groups have a tendency to release electrons which stabilise the positive charge of the intermediate carbocation.

how can alcohols be dehydratedd

with excess hot concentrated sulfuric acid or by passing their vapors over heated aluminium oxide

conditions for partial oxidation of primary alcohol

alcohol in excess, no reflux, more dilute alcohol

conditions for full oxidation of primary alcogol

oxidising agent in excess, reflux

why are tertiary alcohols not readily oxidised

a C-C bond would need to break, as opposed to a C-H bond

what does reflux mean

the vapour condenses and drips back into the reaction flask

what happens when Cr₂O₇^-2 ions are reduced to Cr³⁺

the go from orange to green

What does Tollen’s reagent distinguish between?

aldehydes and ketones

what is the ion in tollen’s reagent

[Ag(NH3)2]+

what happens when the Tollens ion oxidises the aldehyde to a carboxylic acid

the [Ag(NH3)2]+ ions are reduced to a silver mirror

what does Fehling’s solution distinguish between?

aldehydes and ketones

what happens in Fehing’s solution when it oxidises an aldehyde to a carboxylic acid

a blue solution containing Cu2+ ions is reduced to a red-orange precipitate of Cu2O

test + positive result for alkene

shake with bromine water. orange colour disappears.

test + positive result for halogenoalkane

add NaOH(aq) and warm, acidify with HNO3, add AgNO3(aq). precipitate of AgX forms.

test + positive result for alcohols

add acidified K2Cr2O7. orange colour turns green with primary or secondary alcohols (or with aldehydes).

test + positive result for aldehydes

warm with Fehling’s solution/Tollen’s solution. Blue solution turns to red precipitate/silver mirror forms.

test + positive result for carboxylic acids

add NaHCO3(aq). CO2 given off.

what do curly arrows represent in mechanisms

how electron pairs move in organic reactions