halogenoalkanes

not many halogenoalkanes occur…

naturally

what are halogenoalkanes the basis of

many synthetic compounds eg PVC, Teflon and a number of anaesthetics and solvents

what kind of skeleton do halogenoalkanes have

an alkane skeleton with one or more halogen atom in place of hydrogen atoms

general formula of a halogenoalkane with a single halogen atom

CnH2n+1X

often shortened to R-X

how to name halogenoalkane

• halogen prefix tells us which halogen present

• numbers are used if needed to show on which carbon the halogen is bonded to

• prefixes di- tri- tetra- and so on are used to show how many atoms of each halogen are present

• when a compound contains different halogens they are listed in alphabetical order

how are halogenoalkanes named if more than one type of halogen present

ordered in alphabetical order, not in order of the number of the carbon atoms to which they are bonded

bond polarity of halogenoalkanes

• they have C-X bond which is polar

• C partial positive X partial negative

• this is because halogens are more electronegative than carbon

• as you go down group bonds get less polar

why do bonds of halogenoalkanes get less polar down group

electronegativity decreases down the group so not as large a difference in electronegativity

solubility of halogenoalkanes

• polar C-X bond are not polar enough to make them soluble in water

• main intermolecular forces are dipole-dipole attractions and van der waals

• mixed with hydrocarbons so they can be used as dry cleaning fluids and to remove oily stains

what is oil

a mixture of hydrocarbons

what does boiling point of halogenoalkanes depend on

the number of carbon atoms and hydrogen atoms

boiling point trend halogenoalkanes

• increases with increased chain length

• increases going down the halogen group

• these are caused by increased van der waals because the larger the molecules the greater the number of electrons therefore larger the van der waals flrced

as in other homologous series, increased branching of the carbon chain….

will tend to lower the melting point of the halogenoalkane

why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths

• they have higher relative molecular masses

• they are more polar

when halogenoalkanes react which bond is it that almost always breaks

C-X

which two factors determine how readily the C-X bond reacts

• the C-X bond polarity

• the C-X bond enthalpy

how does bond polarity determine how readily the C-X bond reacts

• halogens are more electronegative so C is partial positive and X partial negative

• as C is electron deficient it can be attacked by reagents that are electron rich or have rich areas (nucleophiles)

• polarity of C-X bond would predict that C-F bond would be the most reactive as it is most polar so C+ has the most positive charge and is therefore most easily attacked by a nucleophile

• this means C-I bond is least reactive as least polar

how does bond enthalpy determine how readily C-X bond reacts

• bonds get weaker going down group

• fluorine is smallest atom of halogens and shared electrons in the C-F bond are strongly attracted to the fluorine nucleus giving a strong bond

• going down group the shared electrons in C-X bond get further and further away from the halogen nucleus so bonds become weaker

• bond enthalpies would predict iodo compounds with weakest bonds are most reactive and fluoro compounds with strongest bonds are least reactive

why is bond enthalpy a more important factor than bond polarity in terms of determining how readily the C-X bond reacts

experiments confirm that reactivity increases going down the group, meaning that bond enthalpy is more important than bond polarity

most reactions of organic compounds take place in a series of…

steps

how can you predict steps of reactions of organic compounds

by thinking about how electrons are likely to move

what are nucleophiles

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

charge of nucleophiles

either a negatively charged ion or has an atom with a partial negative charge

a nucleophile has a _____ which it can use to form a covalent bond

lone unshared pair of electrons

where is the lone pair on a nucleophile

on an electronegative atom

in organic chemistry, a nucleophile is a species that has a lone pair of electrons with which it can….

form a bond by donating its electrons to an electron deficient carbon atom

common nucleophile

• :OH-

• :NH3

• :CN-

what happens in nucleophilic substitution reactions

the nucleophile replaces the halogen in a halogenoalkane

nucleophilic substitution reactions all follow essentially the same…

reaction mechanism

what does a reaction mechanism describe

a route from reactants to products via a series of theoretical steps which may involve short lived intermediates

what are curly arrows used to show

how electron pairs move in organic reactions

explanation of mechanism of nucleophilic substitution

• lone pair of electrons of nucleophile is attracted towards partially positive carbon atom, curly arrow starts at lone pair of electrons and moves towards C+

• lower curly arrow shows the electron pair in C-X bond moving to the halogen group making it a halide which is the leaving group

what does rate of substitution depend on

• the halogen

• fluoro-compounds are unreactive due to strength of C-F bond and going down the group the rate of reaction increases as the C-X bond strength decreases

nucleophilic substitution of halogenoalkanes and aqueous sodium/potassium hydroxide

• nucleophile is the hydroxide ion

• reactions occurs very slowly at room temperature

• to speed up, it is necessary to warm mixture

• halogenoalkanes do not mix with water so ethanol used as a solvent in which they mix

• this is called a hydrolysis reaction

• same pattern of curly arrows in mechanism

nucleophilic substitution of halogenoalkanes and cyanide ions

• when warmed with an aqueous alcoholic solution of potassium cyanide, nitriles are formed

• nucleophile is :CN-

• product is nitrile which has one extra carbon in the chain than the starting halogenoalkane

• this is often useful if you want to make a product that has one carbon more than the starting material

• same pattern of mechanism arrows

functional group of nitriles

-C≡N

nucleophilic substitution of halogenoalkanes and ammonia

• reaction of halogenoalkanes with an excess concentration solution of ammonia in ethanol is carried out under pressure

• produces an amine

• ammonia is a nucleophile because it has a lone pair of electrons that it can donate and nitrogen has a partial negative charge

• because ammonia is a neutral nucleophile a proton H+ must be lost to form neutral produce

• H+ ions reacts with a second ammonia molecule to form an NH4+ ion

why are nucleophilic substitution reactions useful

they are a way of introducing new functional groups into organic compounds

what can nucleophilic substitution turn halogenoalkanes into

• alcohols

• amines

• nitriles

• these in turn can be converted to other functional groups

what can a primary amine produced by nucleophilic substitution turn into

• secondary amine

• tertiary amine

what can an alcohol produced by nucleophilic substitution turn into

• aldehyde

• carboxylic acid with same number of carbon atoms as the starting material

what can a nitrile produced by nucleophilic substitution turn into

primary amine with one more carbon atom than starting material or carboxylic acid with one more carbon atom than starting material

how do halogenoalkanes typically react

by nucleophilic substitution

how do halogenoalkanes react if in different conditions

by elimination

what happens in elimination

a hydrogen halide is eliminated from the molecule leaving a double bond in its place so that an alkene is formed

under different conditions OH- ion can act as….

a base, removing a H+ ion from the halogenoalkane

conditions of elimination

• sodium or potassium hydroxide is dissolved in ethanol and mixed with halogenoalkane

• no water present

• mixture is heated

product of elimination

ethene

ethene burns and…

decolourises bromine solution

mechanism of elimination

• :OH- ion uses its lone pair to form a new bond with one of the hydrogen atoms on the carbon next to the C-Halogen bond

• these H atoms are very slightly partially positive

• electron pair from C-H bond now becomes part of C=C

• halogen takes the pair of electrons in the C-halogen bond and leaves as a halide ion (leaving group)

elimination is a useful way of…

making molecules with C=C bonds

why is there competition between substitution and elimination

the hydroxide ion will react with halogenoalkanes as a nucleophile or as a base

if a reaction is substitution or elimination the reaction that predominates depends on what two factors

the reaction conditions (aqueous or ethanolic and the type of halogenoalkane (primary secondary or tertiary)

conditions that favour substitution

hydroxide ions at room temperature, dissolved in water

conditions that favour elimination

hydroxide ions at high temperature, dissolved in ethanol

what do primary halogenoalkanes tend to react by

substitution

what do tertiary halogenoalkanes tend to react by

elimination

what do secondary halogenoalkanes tend to react by

both substitution and elimination

in some cases a mixture of _____ products is possible

isomeric elimination

what are chlorofluorocarbons

halogenoalkanes containing both chlorine and fluorine atoms but no hydrogen

another name for chlorofluorocarbons

CFCs

CFCs reactivity under normal conditions

very interactive

shorter chain CFCs state and uses

• gases

• aerosol propellants

• refrigerant

• blowing agents for foams like expanded polystyrene

longer chain CFCs uses

• dry cleaning

• degreasing solvents

where do CFCs eventually end up

the atmosphere

what do CFCs do once in atmosphere

decompose to give chlorine atoms which decompose ozone in the stratosphere causing a hole in the ozone layer

how have we found out how the ozone has broken down

upper atmosphere research aswell as laboratory research

what is happening to use of CFCs these days

politicians influenced by scientists and under international agreement have decided to phase out and replace CFCs with other safer compounds

example of chemical that is replacing CFCs

hydrochlorofluorocarbons (HCFCs)

why will switching from CFCs not have immediate benefits

a vast reservoir of CFCs remains in the atmosphere and it will be many years before the ozone layer recovers

when and who introduced CFCs

1930s

American engineer Thomas Midgley

what was the original use for CFCs

refrigerators

what did midgley famously demonstrate about CFCs

• their non-toxicity and non-flammability

• to a scientific conference by breathing in a lungful and exhaling it to extinguish a lighter candle

• not long after his death it was realised that CFCs cause ozone depletion

why do HCFCs decompose more easily than CFCs and cause less damage

• due to their C-H bonds

• chlorine atoms are released lower in the atmosphere where they do not contribute to destruction of ozone layer

second generation of replacement of CFCs

hydrofluorocarbons (HFCs)

why do HFCs not damage the ozone layer

they contain no chlorine

why are chemsits working on third generation replacements for CFCs

HFCs are not wholly free of environmental problems

what was used before the advent on CFCs

ammonia