halogens Flashcards
what are halogen alkanes
halogen alkanes have an alkane skeleton with one or more halogen (fluorine, chlorine, bromine or iodine) atoms in place of hydrogen atoms
what is the general formula of halogen alkanes
Cn H2n+1x - formula for a halogenalkane with a single halogen atom and where x is a halogen
This formula is sometimes shortened to R-x
what is the bond polarity
halogen alkanes have a C -x bond
This bond is polar
C-X because halogens are more electronegative than carbon
what is the solubility of halogenalkanes
the polar C-x bonds are not polar enough to make the halogenalkanes soluble in water
what is the main intermolecular forces in halogenalkanes
the main intermolecular forces of attraction are dipole - dipole attractions and van der Waal forces
halogenalkanes mix with hydrocarbons so they can be used as dry - cleaning fluids and to remove only stains (oil is a mixture of hydrocarbons)
what is the boiling points of haloalkanes
the boiling point depends on the number of carbon atoms and halogens atoms
the boiling point increases with increased chain length
boiling point increases going down the halogen group
this is because the increased van der Waals forces because the larger the molecules, the greater the number of electrons (and therefore the larger the van der Waals forces)
why do haloalkanes have higher boiling points than alkanes with similar chain lengths
halogenalkanes have higher boiling points than alkanes with similar chain lengths because they have higher relative molecular masses and they are polar
how do halogenalkanes react
when halogenalkanes react it is almost always the C-X bond that breaks
There are two factors that determine how readily the C-X bond reacts
These are:
- the C-X bond polarity
- the C-X bond enthalpy
what is the bond polarity of halogenalkanes
the halogens are more electronegative than carbon so the bond polarity will be C+-X-
C :X
This means that carbon bonded to the halogen has a partial positive charge - it is electron deficient
This means that it can be attached by reagents that are electron rich or have electron - rich areas.
These are called nuclophiles
what is a nucleophile
a nucleophile is an electron pair donor
which C-X bond is the most polar
the polarity of the C-X bond would predict that the C-F bond would be the most reactivr
It is the most polar, so the C+ has the most positive charge and is therefore most easily attacked by a nucleophile
This argument would make the C - I bond least reactive because it is the least polar
what happens to the bond enthalpies going down the group
C-X bond enthalpies get weaker going down the group
Fluorine is the smallest atomof the halogens and the shared electrons in the C-F bond are strongly attracted to the fluorine nucleus
This makes a strong bond
why does the C-X bond get weaker gong down the group
going down the group, the shared electrons in the C-X bond get further and further away from the halogen nucleus, so the bonds becomes weaker
the bond enthalpies would predict the iodo- compounds with the weaker bonds are the most reactive, and fluro - compounds with thee strongest bonds, are the least reactive
what does reactivity of the haloalkanes depend on
as reactivity increases as you go down the group, it shows that bond enthalpy is more important factor than bond polarity
what are nucleophiles
- are reagents that attack and form bonds with positively or partially positively charged carbon atoms
- a nucleophhilic is either a negatively charged ion or has an atom with a - delta charge
how does a nucleophile form a covalent
a nucleophile has a lone (unshared) pair of electrons which it can use to form a covalent bond
where is the lone pair situated on
an electronegative atom
what are the most common nucleophiles
the hydroxide ion -:OH
ammonia :NH3
cyanide ion -:CN
these will each replace the halogen in a halogenalkane
Thee reactions are called nucleophilc substitutions and they follow essentially the same reaction mechanism
what is the general mechanism for nucleophiles
lone pair of electrons of a nucleophile is attracted towards a partially positively charged atom
the curlyarrow starts at a lone pair of electrons and moves towards C+
The lower curly aroow shows the electron pair in the C-X bond moving to the halogen atom X and making it a halide
The halide is called the leaving group
what does the rate of substitution depend on
the rate of substitution depends on the halogen
e.g. Fluro-compounds are unreactive due to the strength of the C-F bond (high bond enthalpy)
Going down the group, the rate of the rate of reaction increases as the C-X bond strength decreases
how do halogen alkanes react with the nucleophile hydroxide
the nucleophile is the hydroxide ion -:OH
the reaction occurs very slowly at room temps
To speed up the reaction it is necessary to warm the mixture
Halogenalkanes do not mix with water, so ethanol is used as a solvent which the halogenalkane and the aqueous sodium potassium hydroxide both mix
- this is called a hydrolysis reaction
The overall reaction is:
R-X+OH- =ROH+X
so an alcohol ROH is formed
the rate of the reaction depends on the strength of the carbon - halogen bond
Fluroalkanes do not react at all whilst iodoalkanes react rapidly
how is the reaction between halogenalkanes cyanide ions
when halogenalkanes are warmed with an aqueous alcoholic solution of potassium cyanide ion, -:CN
the product is called a nitrile
It has one extra carbon in the chain than the starting halogenalkane
This is often useful if you want to make a product that has one carbon more than the starting material
halogenalkanes with ammonia
the nucleophile is ammonia :NHy
The reaction of halogenalkanes with excess concentrated solution of ammonia in ethanol is carried out under
pressure
The reaction produces on amine, RNH2
ammonia is a nucleophile because it has a lone pair of electrons that it can donate (although it has no negative charge) and the nitrogen atom has a - charge
Because ammonia is a neutral nucleophile a proton, H+, must be lost to form the neutral product, called an amine
The H+ ion reacts with a second ammonia molecule to form an NH4+
what are the uses of nucleophile substitution
nucleophilic substitution reactions are useful because they are a way introducing functyional groups into organic compounds
halogenalkanes can be converted into alcohols, amine and nitriles
These in turn can be converted to other functional groups
primary amine:
secondary amine
tertiary amine
alcohols:
aldehyde
carboxylic acid
nitrile:
primary amine
carboxylic acid
what do halogenalkanes typically react by
typically halogenalkanes react by nucleophilic substitution but under different conditions they react by elimination
what is elimination
a hydrogen halide is eliminated from the molecule, leaving a double bond in its place so that an alkene is formed
in elimination what do OH- ion act as
OH- ion from aqueous sodium or potassium hydroxide is a nucleophilic and its lone pair will attack a halogenoalkane at C+ to form an alcohol
- (an example of nucleophilic substitution)
under different conditions, the OH- can act a base removing an H ion from the halogenalkane
e.g. bromoethane reacts with potassium hydroxide to form ethene
A molecule of hydrogen bromide HBr, is eliminated then the hydrogen bromide reacts with the potassium hydroxide
The reaction produces ethene, potassium bromide and water
what is the mechanism of elimination
- the OH– ion uses its lone pair to form a bond with the hydrogen atoms on the carbon next to the C-Br bond
- these hydrogen atoms are very slightly + - the electron pair from the C-H bond now becomes part of a carbon - carbon double bond
- the bromine takes the pair of electrons inthe C-Br bond and leaves as a bromide ion (the leaving group)
- this reaction is a useful way of making molecules with carbon-carbon double bond
what is the difference between substitution and elimination
since the hydroxide ion will react with halogenalkanes as a nucleophile or as a base, there is competition between substitution and elimination
the reaction that predominates depends on two factors:
- the reaction conditions (aqueous/ ethanolic solution)
- type of halogenalkanes primary, secondary, tertiary)
what does it mean when haloalkanes are: secondary, primary or tertiary
halogens are classified as primary,secondary and tertiary according to whether the halogen atom is at the end of the hydrogencarbon (primary), in the body of the chain (secondary) or at the branch in the chain (tertiary)
this is the same system for alcohols
what are the conditions for elimination
- hydroxide ions at room temp, dissolved in water(aqueous favour substitution)
- hydroxideions at high temp, dissolved in ethanol, favour elimination
what are the isomeric products of elimination
in some cases a mixture of isomeric elimination products is possible
Z-but-2-ene E-but-2-ene but-1-ene
what are CFCs
chlorofluorocarbons are halogen alkanes containing both chlorine and fluorine atoms but no hydrogen, e.g. trichlorofluoromethane, CCl3F
they are also called CFCs
They are very unreactive under normal conditions
what are shorter chain CFCs used for
the short-chain ones are gases and were used e.g.as aerosol, propellant, refrigerants and blowing agents
what are the longer chain CFCs used for
the longer chain ones are used as dry cleaning and d- greasing solvents
why are CFCs bad for the environment
CFC gases eventually end up in the atmosphere where they give chlorine atoms
Chlorine atoms decompose ozone, O3, in the stratosphere, which has caused the hole in the Earth’s ozone layer
the upper atmosphere is broken down
Politicians were influenced by scientists and international agreement,CFCs are being phased out and replaced by other safer, compounds including hydrochlorofluorocarbons HCFCs, CF3, chcl2
However, a vast reservoir of CFCs remains in the atmosphere and it will be many years before the ozone layer recovers
