Further Organic Chem Unit 2, Pt 6
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What is the general formula of alcohols?
Primary, secondary or tertiary alcohol?
- Tertiary (3o)
- Because the hydroxyl (-OH) group is bonded to a carbon which itself is attached to 3 other carbons.
Which type (primary, secondary or tertiary) alcohol is most reactive?
Give 3 methods of how chloroalkane can be made by a substitution reaction with an alcohol.
- With tertiary alcohol - shake alcohol with hydrochloric acid. (because tertiary is very reactive)
- Simplest for all alcohols is to react alcohol with phosphorus pentachloride (PCl5). Products: chloroalkane + POCl3 + HCl. (ad: can be done in room temp. disad: HCl poisonous)
- Alternative is to add conc H2SO4 to alcohol & potassium chloride. KCl + H2SO4 --- HCl + KHSO4. THEN in-situ, alcohol + HCl --- chloroalkane + water
Give method for preparing bromoalkanes. (First one definite, second one need to CHECK with teacher)
- React alcohol with PBr3. Prepare PBr3 in situ by reacting Br with red phosphorus. (like iodoalkanes)
- 50% H2SO4 is needed this time with solid KBr or NaBr and the alcohol.
- KBr + H2SO4 --- HBr + KHSO4THEN, alcohol + HBr --- bromoalkane + H2O
- Conc H2SO4 is NOT used, because it will oxidise the Br- ions in the HBr to bromine.
Give a method for preparing iodoalkanes.
- Even 50% H2SO4 will oxidise HI, so completely new method used.
- Alcohol warmed with red phosphorus and iodine. First, phosphorus reacts with iodine to make phosphorus triiodide (PI3) which attacks alcohol.
- 2P + 3I2 --- 2PI3
- THEN, alcohol + PI3 --- 3 iodoalkane + H3PO3.
How do you test for the presence of -OH group?
- Add PCl5 (phosphorus (V) chloride) to the unknown liquid.
- If -OH present, you'll get steamy fumes of HCl gas, which will turn moist blue litmus red.
- HCl also dissolves in water to form chloride ions. You can then test for Cl- ions by using silver nitrate.
- Equation: ROH + PCl5 --- RCl + HCl + POCl3
Describe the reaction between alcohol and sodium.
- Sodium breaks the O-H bond to produce ionic alkoxides.
- 2CH3OH + 2Na --- 2CH3O-Na+ + H2The longer the hydrocarbon chain on alcohol, the less reactive it is with sodium.
Why do alcohols have higher boiling temperatures compared to alkanes of similar sizes?
Alcohols have a higher boiling temperature, because the OH group on alcohols help them to form hydrogen bonds which is the strongest intermolecular force.
Describe the trend of the miscibility of alcohol in water as its size gets larger. Why?
- Small alcohols are more miscible in water than larger ones.
- Because hydrogen bonds can form between the -OH and H2O. In larger alcohols, most of the molecule is a non-polar carbon chain, so there's less attraction for polar water molecules.
- (small alcohols also miscible in some non-polar solvents like cyclohexane)
Describe how alcohols react with oxygen.
- Forms carbon dioxide and water.
- Burn with pale blue flame.
- The C-H and C-C bonds are broken as they are completely oxidised.
How much an alcohol can be oxidised depends on its structure. For each type of alcohol, name what is produced when oxidised.
- Primary - oxidised to aldehydes and then to carboxylic acids.
- Secondary - oxidised to ketones only.
- Tertiary - won't be oxidised.
Aldehydes and ketones are both carbonyl compounds. What is the difference in structure?
- Aldehydes have a hydrogen and one alkyl group attached to carbonyl carbon atom.
- Ketones have two alkyl groups attached to the carbonyl carbon atom.
What substance can be used to oxidise alcohols?
- acidified potassium dichromate (VI) (acidified, so in sulfuric acid)
- (this is the oxidising agent)
How do you oxidise primary alcohols to produce aldehydes?
- Gently heat excess alcohol with controlled amount of oxidising agent (potassium dichromate (VI) in distillation apparatus, so the aldehyde (which boils at lower temp than the alcohol) can be distilled off immediately before being further oxidised to carboxylic acid.
- (Water is also produced!)
How do you oxidise primary alcohols to produce carboxylic acid?
- Alcohol has to be vigorously oxidised.
- Alcohol mixed with excess oxidising agent (potassium dichromate (VI) ) and heated under refulx.Heating under reflux means you can increase temp of an organic reaction to boiling point without losing volatile substances. Any vaporised compounds are cooled, condense (because of Liebig condenser) and drip back into mixture.
- Round-bottomed flask, with anti-bumping granules to make boiling smoother, and Liebig condenser.
How do you oxidise secondary alcohols to get ketones?
- Reflux with excess acidified potassium dichromate (VI).
- But ketones are not oxidised easily.
How do you know when the dichromate (VI) ion reacted with the alcohol? (in primary in secondary)
- Dichromate (VI) ion is orange but when it is reduced, it goes to green chromium (III) ion, Cr3+.
- (In tertiary alcohols, solution stays orange, since it doesn't react with dichromate).
What can you do to distinguish between aldehydes and ketones? (2 methods)
- Fehling's solution or Benedict's solution: deep blue Cu2+ complexes, which reduce to brick-red Cu2O when warmed with aldehyde, but stay blue with ketone.
- Tollen's reagent: reduced to silver when warmed with an aldehyde, but not with ketone. Silver will coat inside of apparatus and form a silver mirror.
- (Because aldehydes are oxidised easily, but ketones aren't)
Halogenoalkanes can be primary, secondary or tertiary. Describe each.
- Primary: Carbon with halogen attached has - 2 hydrogen atoms and 1 alkyl group. (on the end therefore)
- Secondary: Carbon with halogen attached has - 1 hydrogen atom and 2 alkyl groups.
- Tertiary: Carbon with halogen attached has no hydrogen atoms and 3 alkyl groups.
Describe the experiment in which you can compare the reactivity of primary, secondary and tertiary halogenoalkanes.
- Firstly, important to note that mixing halogenoalkane with water forms alcohol, H+ ions, and halide ions. Silver nitrate will make this halide ion form a silver halide precipitate. You can measure how fast precipitates form with 3 of the halogenoalkanes (primary, secondary or tertiary).
- So, set up 3 test tubes with 3 different halogenoalkane (1o and 2o and 3o), ethanol (as solvent) and silver nitrate solution (containing water).
- RESULTS: tertiary haloalkane forms precipitate immediately; secondary forms precipitate in several seconds; primary form in several minutes.
- THUS tertiary is MOST REACTIVE.
What are some of the uses of halogenoalkanes?
- Used as modern refrigerants. Used to cool air in fridge or air conditioning system. - Suited for this because they are easily compressible, non-toxic, gases at room temp.
- Some non-flammable are used as fire retardants or flame retardants. (eg. synthetic fibres in children's pyjamas)
- CFC's used to be used for a lot of things - but now its risks outweigh the benefits for it damages the ozone layer - so banned. HCFC's are used in place of CFC's now (less damaging to ozone layer because its less stable and will decompose before)
Give 2 examples of nucleophiles that react readily with halogenoalkanes.
What is the name of the reaction where a nucleophile reacts with a slightly positive carbon atom?
Why can halogenoalkanes react by nucleophilic substitution?
- Halogens are much more electronegative than carbon, so carbon-halogen bond is polar.
- The slightly-positive carbon is attacked by a nucleophile (which is an electron-pair donor).
- The nucleophile bonds with the carbon and is substituted for the halogen. The C-X (X is halogen) bond breaks heterolytically.
- (3 examples you need to know!)
Describe reactions of halogenoalkanes with aqueous alkalis.
- Forms alcohol and Br- ion.
- OH- is substituted for the halogen - aqueous alkali provides this.
- It is a nucleophilic substitution reaction with hydrolysis reaction (reacted with water).
- Has to be warm aquous sodium/potassium hydroxide under refulx or it won't work.
What are the 3 nucleophilic substitution reactions that I need to know for halogenoalkanes (according to revision book).
- Reaction with aqueous alkalis - form alcohols.
- Reaction with water - form alcohols.
- Reaction with ammonia - form amines.
Describe reaction between halogen and water.
- Water is weak nucleophile - but eventually will substitute for halogen, even though its much slower than with aqueous alkali.
- Alcohol formed as well as H+ and Br-
- An intermediate is formed with oxygen that has 3 bonds - unstable, so one O-H bond breaks.
Describe reaction between halogenoalkanes and ammonia.
- Form amines - CH3CH2NH2 + NH4Br
- Warm haloalkane with excess ethanolic ammonia, and ammonia (NH3 - another nucleophile) substitutes for the halogen. (Under heat and pressure)
- An intermediate is formed CH3CH2NH3, but then another ammonia molecule removes a hydrogen to form NH4+. This NH4+ combine with Br- ion to form ammonium bromide.
Halogenoalkanes can react with aqueous alkali and alcoholic alkali. What are the products for each one and what are the reactions?
- with aqueous alkali - alcohol formed [Nucleophilic substitution]
- with alcoholic alkali (alkali dissolved in alcohol such as ethanol) - alkene formed [Elimination reaction]
Describe reactions between halogenoalkane and alcoholic alkali.
- Alkene, water and Br- ions (which might combine with K or Na or something) is formed.
- Elimination reaction - H and Br on haloalkane are lost and aren't replaced.
- OH- attacks a carbon, carbon has electrons and thus makes double-bond with other carbon, so Br heterolytically breaks with C to become ion.
- Heat gently.
Write equation down for the nucleophilic substitution between halogenoalkanes and aqueous silver nitrate.
- CH3CHBrCH3 + H2O + Ag+ ---- CH3CH(OH)CH3 + AgBr + H+
- (This is the same as the reaction with water, except there's a silver ion in it which reacts with halogen to form precipitate).
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