Topic 6 organic chemistry part 2 Flashcards
How do you name halogenoalkanes?
1) Find the longest carbon chain (will form the last part of the name)
2) The names and positions of the halogen on the molecules comes first (prefix)
3) The prefixes are fluro, chloro, bromo. Use numbers to state their positions on the carbon chain
4) If you have more than 1 of the same halogen prefix di, tri, tetra
Reaction of halogenoalkanes with water (nucleophilic substitution)
Haloalkanes react with water via nucleophilic substitution
Heat and water is required.
C2H5Cl + H2o —-> C2H6O + H+ + Cl-
Reactivity of halogenoalkanes
They become more reactive as we go down the group and hydrolyse the fastest
The bond strength determines the reactivity. The halide ion gets larger down the group as this leads to a weaker bond.
C-F is strongest and C-I is weakest
What experiment shows the reactivity of halogenoalkanes?
Place chloro bromo and diodo alkane into 3 test tubes
Add silver nitrate solution and ethanol solvent to each tube
Iodide - yellow precipitate forms first
Ag++ I- —-> AgI
Bromide - Cream precipitate is formed second
Ag+ + Br- —-> AgBr
Chlorine —->
Ag+ + Cl- —-> AgCl
Primary halogenoalkanes react the slowest
Tertiary reacts the fastest. Add water and then silver nitrate
What is bond polarity and nucelophiles?
Halogenoalkanes have a polar bond and are attacked by nuclephiles
Halogens are more EN than carbon so they pull electrons towards themselves in a covalent bond. This leads to a polar bond
This polar bond means that halogenoalkanes can be attacked by nucelophiles.
A nucleophile is a substance that is an electron pair donor to something that is delta+ or deficient in electrons
Examples of nucleophiles are CN- NH3 and OH-
Halogenoalkanes reaction with OH- (nucleophilic substitution)
Need warm aqueous NaOH - water can be used but the reaction would be much slower
Carried out under reflux
Halogenoalkanes reaction with CN-
They make nitriles
Conditions are warm ethanoic CN- ions and carry out under reflux as we don’t want them evaporating
Halogenoalkanes reaction with NH3
Conditions are heat with ethanoic acid and must have excess ammonia.
An amine is produced (identified by a fishy smell)
Ammonium ion is also produced
Halogenoalkanes reaction with OH- ion (elimination)
OH- will attack hydrogen on a carbon adjacent to the carbon with the halogen on
OH- acts as a base forming water
Alkene is formed and water and Kbr
How to tell if a reaction is elimination or substitution?
The solvent plays an important role in deciding
When reacting NaOH with a halogenoalkane we can make:
An alkene - when using ethanol as a solvent (elimination) - when OH- acts as a base
An alcohol - when using water as a solvent (substitution) where OH- ion acts as a nucleophile
What is the general formula of an alcohol?
CnH2n+1OH
OH group is called a hydroxyl group
Tertiary alcohols react fastest primary alcohol react slowest
How to make Chloroalkanes?
Made by reaction alcohol with PCl5 or Hcl
Using Pcl5:
ROH + Pcl5 —-> Rcl + Hcl + Pocl3
Using Hcl:
ROH + Hcl —–> ROH + Hcl —-> Rcl + H2O
Tertiary alcohols react to fastest with Hcl and slowest with primary alcohols
How to make bromoalkanes?
Haloalkanes are made from alcohols via substitution reaction
A bromoalkane is made when an alcohol is reacted with Br- source (NaBr)
An acid catalyst (50% conc H2SO4) is used
Initially - NaBr reacts with H2SO4 to form Hbr
Then Hbr reacts with alcohol to form bromoalkane + water
How to make iodoalkanes?
They are made by reacting an alcohol with PI3
3ROH + PI3 —-> 3RI + H3PO4
PI3 is mad in situ under reflux with the alcohol red phosphorus and iodine
What is the dehydration of alcohols?
Alkenes are made by the dehydration of alcohols
Conditions - the use of H2SO4 or H3PO4
For non primary chains longer than 3 carbons, you get different positional isomers and may also get E/Z isomerism
What is the oxidation of alcohols?
They can be oxidised to aldehydes, ketones and carboxylic acid.
They are oxidised using acidified potassium dichromate. This is a mild oxidising agent so is reduced itself. It will turn from orange (dichromate ion) to green (chromium ion)
Primary alcohols can be oxidised to aldehyde then carboxylic acid
Secondary can be oxidised to ketone
Tertiary can’t be oxidised
How to get aldehyde and carboxylic acid from primary alcohols
Use distillation as aldehyde has lower BP than alcohol
Use reflux and excess oxidising agent to make carboxylic acid. Reflux allows strong heating without losing volatile reactants and products. Aldehydes evaporate, condense and fall back into the flask.
How to get krtones from secondary alcohols
Use reflux and an oxidising agent to make a ketone
How to test for aldehydes and ketones?
Fehlings and Benedicts is an oxidising agent and so oxidises aldehydes but not ketones
Fehlings solution is blue as it contains Cu2+ ions. Add to water bath to heat.
Aldehydes - Fehlings solution goes from blue solution to brick red precipitate
Ketones - Remains blue
What is reflux?
It is a technique used when you want to heat a volatile liquid
Reflux allows strong heating without losing volatile reactants and products. Volatile compounds evaporate and condense and fall back into the flask
The liebig condenser has cold water running through the tube. When hot evaporating substances hit the holder condenser they turn back into liquid and return back to the round bottomed flask to react further.
As we are using flammable liquids, heating is done via a water bath or electric heater called a mantl. This is safer than using a naked flame
What is distillisation?
Used when we want to separate substances with different BP’s.
Gently heating a mixture will result in the compounds separating out in order of BP.
Knowing the BP of the chemical you want to separate will allow you to decide how you separate your compound
If your compound has a lower BP than your starting mixture, you heat to the temp of your BP of your compound if you want to separate. Collect your product in a separate vessel
If your compound has a higher BP than your starting mixture, you heat to the temp of the BP of the compound you want to separate. Your compound will remain at the bottom of the flask
