Halogenoalkanes Flashcards
Chloroform is part of a ?
halogenoalkane
What is a halogenoalkane
It’s an alkane where one molecule is replaced by a halogenoalkane
When a molecule is an alkane we write what?
so what do we name this alkane ?
“Ane” at the end of the name
methane
What is this molecule called?
Fluoromethane
what’s the name of this molecule?
bromoethane
What is this molecule called
1-bromopropane
What is this molecule called
2-bromopropane
what do we call this molecule
2-chlorobutane
What do we call this molecule
and what does the position number tell us
3-fluoropentane
the 3 is the position number of the fluorine and tells us where the halogen is located in the carbon chain
What is this molecule called
1-fluorobutane
what do we call this molecule
2-iodohexane
What is the general formula for halogenoalkanes?
CnH2n+1
What halogenoalkane is this molecule?
secondary as the carbon bonded to chlorine is bonded to 2 more carbons
What type of halogenoalkane is this
tertiary as the carbon bonded to chlorine is bonded to 3 more
What type of halogenoalkane is this
primary as the carbon bonded to Chlorine is bonded to a single carbon
All halogenoalkanes are…
as the length of the halogenoalkane carbon chain increases…
polar molecules
meaning they experience van der waals forces and dipole dipole forces
as the length increases the van der waals forces increases
are halogenoalkanes soluble in water?
Halogenoalkanes are only slightly soluble in water
as the length of the halogenoalkane increases we have more C-C bonds and C-H bonds which are non polar so solubility increases as non polar substances are highly insoluble in water
what is a nucleophilic substitution?
Where a nucleophile e.g cyanite, hydroxide etc is substituted for a halogen
what’s happening here
Halogenoalkanes are polar so the Br molecule has a delta negative charge whereas the carbon bonded to bromine has a positive one
The hydroxide ion has a lone pair and that line pair is attracted to the Positive carbon bit so the whole molecule acts as an electrophile (proton acceptor / base ) meaning the lone pairs arrow goes from the lone pair to the carbon
What’s happening here
Since a new bond is formed between the carbon and the hydroxide ion,
another bond has to break as carbon can only make 4 bonds
the carbon bonded to bromine breaks
making an arrow from the bond pointing to bromine making bromine be substituted for OH like this
What’s the structural formula (Ionic equation) of this molecule
CH3CH2CH2Br + OH- —> CH3CH2CH2OH + Br-
what’s the structural formula (full equation) of this equation?
CH3Cl + OH- + Na+ —> CH3OH + NaCl
full equation =
CH3Cl + NaOH —> CH3OH + NaCl
What is the reagent and nucleophile in this reaction?
OH- = nucleophile
NaOH is the reagent
What is the reagent and nucleophile in this reagent?
KCN (potassium cyanide) is the reagent
whereas CN- (cyanite) is the nucleophile
what is the reagent and nucleophile in this reaction
NH3 is the reagent
and NH3 is the nucleophile
What is a nucleophile and reagent?
Nucleophile = donates an electron pair to form a covalent bond
reagent= substance that is physically added to the system
Identify the reagent and nucleophile
Nucleophile = CH3CH2O Reagent = CH3CH2ONa
Deduce what products would form and the arrow
What is the product called?
OH points from its line pair to the middle C bonded to Br & C-Br bond breaks substituting OH for Br and making it a single Br- product
the OH on the molecule is called an alcohol
so when an hydroxide forms a nucleophilic substitution reaction is always forms an alcohol
In hydroxide reactions in nucleophikic substitution we can use NaOH and KOH to make our alcohol.
What are they examples of?
Reagents
In order for the halogenoalkanes to undergo nucleophilic substitutions with hydroxides (-OH) the reagents need to be…
(NaOH & KOH) Reagents need to be in the aqueous state
they need to be dissolved in water
Increasing the temperature…
increases the rate of reaction so chemists heat the reaction with the reagents (NaOH & KOH) to get faster results
What would form?
CN- would form a bond with the C bonded to I breaking the bond with I
so I would be substituted for CN forming also I-
The cyanide ion behaves weird. Carbon behaves as a negative ion although technically Nitrogen is more electronegative giving it a negative charge so both C and N have lone pairs
What happens in this reaction
also what homologous series is this in?
1) write delta positive and negative charges
2) CN- forms a bond with C bonded to Br breaking also the C-Br bond replacing CN with Br with products being Br-
it’s in the nitrile homologous series
halogenoalkanes undergo nucleophilic substitution with cyanites to form nitriles
What is this molecule called
- ethanenitrile
- propanenitrile
- butanenitrile
To produce nitriles we can heat what?
What are the other conditions required to produce a nitrile as well in nucleophilic subsitution?
heat the reagent KCN ( potassium cyanide) or NaCN (sodium cyanide)
heating the reaction increases the rate of reaction
dissolving NaCN and KCN in liquid ethanol making it nitriles rather than alcohols
1-iodopropane undergoes nucleophilic substitution with sodium cyanide
complete the equation for this reaction..
CH3CH2CH2CHCI + NaCN —> CH3CH2CH2CHCN + NaI
Why do we need to dissolve the KCN and NaCN in ethanolic liquid solution? why not just water
if we dissolve the solution in water it will turn into an alcohol
as the OH in water can act as a nucleophile and react
if we dissolve it in liquid ethanol there won’t be hydroxide ions and ensure the cyanide ions react
How many mechanism steps does ammonia have?
2
Ammonia has these steps explain them
ammonia lone pair forms a bond with the carbon bonded to halogen (Cl) breaking the C-Cl bond replacing the NH3 making it a + ion as it gained electrons
to make it to NH2 we add another ammonia nucleophile and react it’s lone pairs with 1 hydrogen and breaking the bond of hydrogen it’s reacting with nitrogen forming NH2 and NH4Cl
Why doesn’t the nitrogen form a bond with ammonia? because it’s more electronegative forming a delta neg charge and allowing the H to be positive it forms a bond with the lone pair of NH3
Write the equation for this reaction
CH3CH(Br)CH3 + 2NH3 —> CH3CH(NH2)CH3 + NH4Br
What’s the overall equation for the reaction between 1-chloroethane and ammonia to form an amine?
CH3CH2Cl + 2NH3 —> CH3CH2NH2 + NH4Cl
To form animes, we react halogenoalkanes with
ammonia (NH3) and dissolve it in ethanol
and also form amines from halogenoalkanes by heating the reaction, using high pressures and using excess ammonia
The rate of reactions of halogenoalkanes is determined by
bond enthalpy
so the lower the bond enthalpy the fast the reactions
which in this case halogens, iodine has the lowest bond enthalpy so iodine reacts faster
How do you measure the reactivity of halogenoalkanes?
Dissolve the halogenoalkane in ethanol
then add silver nitrate (AgNO3) and water and measure the time it takes for a precipitate to form
the quicker the precipitate forms the faster the rate of reaction
What is an elimination reaction?
2 things occur,
1) 2 sigma bond break
2) a pi bond forms
e. g
if we react 2-bromopropane with NaOH (aq) we get…
an nucleophilic substitution reaction where the product alcohol is formed making it propanol and NaBr
But if we react 2-bromopropane with ethanolic sodium hydroxide NaOH instead we get…
an elimination reaction producing an alkene
What is the trend following to get an elimination reaction and nucleophilic substitution?
If we react any halogenoalkane with an aqueous hydroxide (NaOH/KOH etc.) these reactants undergo an nucleophilic substitution reaction and produce an alcohol
but when we react that same halogenoalkane with ethanolic hydroxides they undergo elimination reaction producing an alkene
What’s produced in the following reactions?
a) 1-iodobutane reacts with ethanolic potassium hydroxide
b) 1-iodobutane reacts with aqueous sodium hydroxide
a) . elimination reaction producing an alkene
b) nucleophilic substitution producing an alcohol
How do we increase the rate of reaction of elimination reactions?
heat the mixture
In a nucleophilic substitution reaction state the conditions
the reagent (NaOH/KOH etc) needs to be aqueous the reagent needs to be diluted and in lower temperatures
in exam you need to state that it needs to be in aqueous and lower temperature to increase the chances of nucleophilic substitution occurring
in elimination reaction state the conditions
reagent needs to be dissolved in ethanolic solution
needs to be concentrated (NaOH/KOH)
they need to be in high temperatures as well to increase the chances of elimination occurring
The brønsted lowry model defines a base as an
proton acceptor
depict what is acting as a base
1) HO isn’t acting as a base as it’s forming a bond meaning it’s donating it’s electrons
2) HO acting as a base as its accepting a bond which is broken
another example
water first isnt acting as a base as its forming a bond
water in second is accepting a H+ ion broken from O so it’s acting as a base
What is the role of -OH in this reaction?
OH is acting as a base
How do we represent with curly arrows 2 sigma bonds breaking and a oi bond breaking in this example
1) arrow tells that the hydroxide line pair forms a bond with hydrogen where the hydroxide ion acts as a base
2) arrow telling us the carbon hydrogen bond is breaking and a pi bond is forming where the arrow is pointing
3) arrow tells us the carbon bromine bond is breaking
but the the carbon hydrogen bond doesn’t break on its on. for it to occur (elimination) we need to dissolve out hydroxides in ethanol
What are the final products?
alkene and water and the halogen
explain what’s happening here
1) hydroxide ion forms a bond with hydrogen breaking the bond with carbon
2) carbon iodine bond breaks making C=C alkene making water and I-
Which hydrogen is removed in elimination reaction
in this reaction
we can only remove the hydrogens next to Iodine and below highlighted
what’s the trend of removing hydrogen
we can only remove hydrogens adjacent to the halogen
Show us an example of why you can remove 2 and still get the same product
hydrogens can be removed carbon adjacent to the carbon halogen bond
Identify the error of this elongation reaction drawn
1) Hydroxide ion is missing a lone pair
2) the curly arrow from C—C to C—H should be reverse like this
What products do we get if we react ethanolic sodium hydroxide with 2-chlorobutane?
depending on where the hydrogen is removed we can get 2 products
1) if we remove the hydrogen from the first carbon we get but-1-ene
2) if we remove a hydrogen from the third carbon we get but-2-ene
both of these are examples of structural isomers as they have the same molecular formula but different structural formulas
Alkenes have restricted rotation around their double bond. This means that if an alkene has 3 substituents, it will have
E-Z isomers
2 substituents being CH3
so the reaction produces both E and Z isomer
bcs it has 2 substituents we get E-but-2-ene and Z-but-2-ene
How do these isomers look like
E-but-2-ene and Z-but-2-ene
1) substituents being on the opposite sides so it’s E but 2 ene
2) substituents in the same side so it’s a Z but 2 ene
What products do we get in 2-chlorohexane
we get Hex-1-ene and hex-2-ene
and hex-2-ene E/Z isomers
how do we write the equation from this reaction?
CH3CH2CH2Br + KOH —> C3H6 + KBr + H2O
ionic
CH3CH2CH2Br + OH- —> C3H6 + H2O + Br-
since the hydrogen can only be removed from the second carbon as that’s the one adjacent so it can’t produce any position isomers so we write the molecular
formula
Write the equation for this reaction
because we are producing different position isomers we need to write the structural formula
1) ionic
2) full equation
