2.10 Halogenoalkanes Flashcards by capriconi j

halogenoalkanes can be classified into

primary

secondary

tertiary

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CH3 CH2 CH2 CH2 CH2 I

1 - iodopentane

primary

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CH3 CH2 CH2 CH2 CH(Br) CH3

2 - bromopentane

secondary

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(CH3)2 C Cl CH2 CH3

2 - cloro - 2 - methylbutane

tertiary

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why carbon–chlorine bond is polar

it has more electrons and chlorine atom is very electronegative

carbon atom is slightly positive

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why halogenoalkanes attacked by nucleophiles

nucleophiles attack slightly+ C atom in the carbon — halogen bond

the halogen atom gains electrons from the sigma bond and forms halides

attract electron rich nucleophiles

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reactions of halogenoalkanes

name 3

elimination

substitution ( nucleophilic )

hydrolysis

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elimination reaction
hydrogen atom is removed from carbon next to the one bonded to halogen

product is ?

alkene

or a mixtures of alkene

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elimination reaction

eq
reagents
condition
product
reaction type

eq: refer to notes pg 2
reagents: conc potassium / sodium hydroxide
condition: heat under reflux with ethanol
product: propene ( reactant : 1 - bromopropane )

reaction type: elimination

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elimination reaction

mechanism

draw it out

refer notes at pg 2

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elimination reaction

draw diagram for heat under reflux

to prepare propene from 2 - bromopropane

refer notes pg 2

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elimination reaction

why need to distill the mixture

to remove volatile product as it forms

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elimination reaction

why condenser is not used in heat under reflux

propene has very low boiling points

will not condense at room temp

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substitution reaction

halogen is substituted by an -OH, producing an alcohol

eq
reagents
condition
product
reaction type

eq : refer to notes

reagents: dilute potassium / sodium hydroxide
condition: heat under reflux in aq solution in ethanol
product: propanol ( reactant: chloropropane )

reaction type: nucleophilic substitution / hydrolysis

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substitution reaction

why halogenoalkane is dissolved in alcohol
and alkali dissolved in water?

as a solvent for both aq and halogenoalkane

to allow halogenoalkane and water to mix

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elimination and substitution

compare

conditions
type of reactions
action of OH-
product

conditions: reflux with aq solution of KOH

type of reactions: nucleophilic substituion

action of OH- : nucleophilic

product: alcohol

2.
conditions: reflux with alcoholic solution of KOH

type of reactions: elimination

action of OH- : base

product: alkene

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2- chlorobutane in ethanolic aolution gives 3 diff alkenes

name and draw displayed formula

refer to notes pg 3

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mechanism of nucleophilic substitution

2 types when hydroxide ions attack a halogenoalkane

SN2

SN1

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the mechanism of nucleophilic substitution depends on?

primary secondary or tertiary halogenoakanes

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SN2 mechanism for?

primary and secondary

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SN2 mechanism

draw the mechanism

involves?

transition state

single step mechanism

refer notes pg 4

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reaction profile of SN2

refer notes pg 4

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when does transition step occurs ?

in SN 2

refer to notes pg 4

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the rate of order of SN2

idoalkane

bromoalkane

chloroalkane

C–I bond is weakest reaction faster

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SN1 mechanism for

tertiary and secondary halogenoalkanes

SN1 mechanism draw the mechanism involves?

carbocation intermediate two steps refer to notes pg 5

SN1 mechanism reaction profile

refer notes pg 5

two reasons why SN1 is more favoured by tertiary halogenoalkane

1. more stable tertiary carbocation intermediate is formed 2. presence of two alkyl groups which are bulky attached to slightly+ C of C--X bond inhibit approach of OH- effect is called as steric hindrance

can primary halo undergo SN1

formation of primary carbocation has higher activation energy rate of reaction is slower

increase conc of NaOH will affect rate of which mechanism?

SN2 as high conc of nucleophile are requireed for the slowest step to speed up the reactoin

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia halogen is substituted with -NH2, producing amine ``` eq reagents condition product reaction type mechanism ```

eq: refer to notes pg 6 reagents: excess ammonia in ethanol condition: heated in a sealed tube product: ethylamine ( reactant: chloroethane ) reaction type: nucleophilic substitution mechanism: SN2

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia draw the mechanism

refer to pg 6

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia why reaction is carried out in a sealed tube rather than heat under reflux

ammonia gas would escape it would not be condensed by reluc condenser boiling point 33.43 degree celcius

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia alternative method to heating in sealed tube

conc solution of ammonia at room temp leave for a long time to increase rate of reaction

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia why ammonia is used in excess

there is 2 nucleophiles present - ethylamine - NH3 competition btwn them so excess ammonia -to reduce the proportion of further substitution ethylamine has line pairs of electrons can act as nucleophiles

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia why ammonia chloride salt is formed instead of HCL

NH3 is in excess when react with HCL forms ammonia chloride

further reactions of halogenoalkanes 1. reaction with alcoholic ammonia even in presence of alcoholic conc NH3 no elimination taek place only nucleophilic substitution why?

NH3 is a weak base ( slow reaction ) but a strong nucleophile and attack slightly + C in C---X bond OH- strong base, weak nucleophile

further reactions of halogenoalkanes 1. reaction with water ( hydrolysis ) to study the reactivity of halogenoalkanes the experiment procedure?

refer notes on G7

further reactions of halogenoalkanes 1. reaction with water ( hydrolysis ) same primary halogenoalkane rate of hydrolysis explanation

refer notes on pg 7

further reactions of halogenoalkanes 1. reaction with water ( hydrolysis ) primary, secondary, tertiary alcohol rate of hydrolysis explanation

refer notes pg 7

test for halogenoalkanes experiment procedure?

halo is warmed with some NaOH in ethanol and water the mixture is acidified by adding dilute HNO3 Ag NO3 solution i added

chloroalkane bromoalkane iodoalkane observation with AgNO3 and ionic eq

refer notes pg 8

test for halogenoalkane halo is warmed with some NaOH in ethanol and water the mixture is acidified by adding dilute HNO3 Ag NO3 solution i added is actually difficult to distinguish the colours. what can you add to confirm each of these precipitate?

add NH3 AgCl dissolve in dilute NH3 AgBr insoluble in dilute NH3 but is soluble in conc NH3 AgI insoluble in dilute and conc NH3

test for halogenoalkane halo is warmed with some NaOH in ethanol and water the mixture is acidified by adding dilute HNO3 Ag NO3 solution i added why NaOH was added

to allow nucleophilic substitution to hydrolyse the halogenoalkanes and liberate the halides

test for halogenoalkane halo is warmed with some NaOH in ethanol and water the mixture is acidified by adding dilute HNO3 Ag NO3 solution i added why the solution has to be acidic prior to addition of AgNO3?

neutralise NaOH prevent from getting grey percipitate of AgOH from KOH when AgNO3 is added

IUPAC naming CFC - 11

trichlorofluromethane

IUPAC naming CFC - 123

1,1,2 - trichloro - 1,2,2 trifluroethane

properties of CFCs ( no hydrogen atom )

inert non flammable non toxic odourless / tasteless volatile

uses of CFCs

refrigerants - cooling liquid aerosol propellants - perfume and insecticides spray cleaning agent - solvent in dry cleaning and electronic industries flame retardant - fire extinguisher

CClBrF2 fire retardant

non flammable inert due to strong C--F bond forms a protective layer prevent underlying material from igniting and exclude O2 halogenoalkanes absorbs heat from fire absorb free radicals in combustion propagation

CCl2F2 as refrigerant

inert low boiling point vapourise rapidly at low temp

inertness and stability of CFCs has a disadvantge

tend to persist in the atmosphere for long periods destroy ozone layer

structure of ozone layer

refer to note pg 10

the initiation , propagation eq when UV strikes a CFC molecule producing free radicals

refer to note pg 10

why chlorine radicals act as catalyst? the consequences?

regenerate so small amount can destroy thousand of ozone molecules before termination step

replacement of CFCs called hydroclorofluorocarbon which contain hydrogen atoms advantages?

more reactive broken down in the lower part of atmosphere and pose a lower risk to ozone layer

replacement of CFCs called hydroclorofluorocarbon which contain hydrogen atoms disadvantges

greenhouse gas flammable formation of photochemical smog

HCFC - 22 CHClF2 naming

chlorodifluromethane

HCFC - 123 CF3CHCl2 naming

2,2 - dicloro - 1,1,1 - trifluoroethane

why ozone is bad near earth s surface

harmful to respiratory systems

why ozone is good at stratosphere

absorb Uv reduce skin cancer

why ozone is preferred than chlorine as a disinfectant

1. ozone involves in a reversible reaction turns to pure o2 when unused leaves no harmful by products ozone oxidation faster than chlorine 2. chlorine is acidic in water has a taste / unpleasant odour

cars produce nitrogen oxide ( free radical ) but aircraft causes more damage why?

nitrogen oxides produce from cars are oxidised to NO2 when it rains it produced HNO3 acid rain NO never gets a chance to reach stratosphere aircraft fly closer to upper atmosphere, NO can reach stratosphere

NO breaks down ozone two propagation steps

refer notes pg 11

CCl4 is a good fire retardant but pose risk to ozone CBrF3 is not as effective but has the smallest environmental impact why ?

1. C --- F in CBrF3 has the strongest bond it takes a lot of energy to break to form radicals C---F causes global warming reach stratosphere never breaks down 2. Br : F 1 : 3 although C -- Br can be broken, 3F is still there cannot be broken 3. C---Br is weaker easier to be broken before reaching the stratosphere

2.10 Halogenoalkanes Flashcards

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