Compounds Containing the Carbonyl Group Flashcards
How do you distinguish between aldehydes and ketones?
(state simple chemical test; why this occurs)
These tests work on the fact that aldehydes can be easily oxidised to a carboxylic acid, but a ketone can’t.
Tollens’ test (silver mirror) [Ag(NH3)2]+
- Gentle oxidising agent; solution of silver nitrate in aqueous ammonia.
- (Colourless silver(I) complex ions) are reduced to silver when warmed with an aldehyde (aldehyde readily oxidises to carboxylic acid); silver coats inside of the apparatus to form a silver mirror.
- Has no effect with ketone.
- Once used commericially for making mirrors.
Ag(NH3)2+ + e- → Ag + 2NH3 (silver is reduced)
Colourless Silver
RCHO + [O] → RCOOH (the aldehyde is oxidised)
Benedict’s (Fehling’s) test
- Gentle oxidising agents; both contain blue copper(II) Cu2+ complex ions.
- These oxidise aldehydes (reduced themselves) going from a deep blue to a green intermediate colour then giving a brick-red precipitate, but not with ketones, staying blue.
Cu2+ + e- > Cu+
What are the hazards of synthesis using HCN/KCN?
(for nucleophilic addition to form hydroxynitriles)
- HCN, hydrogen cyanide, is a highly toxic gas; to reduce the risk, a solution of acidified potassium cyanide (KCN/H+;H2SO4 little) is used instead.
The cyanide ions needed for the reaction are formed in the solution. Even so, the reaction should be done in a fume cupboard.
* Also, the organic prodcuts are flammable; if you heat them, use a water bath or electric mantle, rather than a flame. (applies to aldehydes/ketones as well)
How can you form alcohols from aldehydes and ketones?
Propanone + (Reagent)
(mechanism + conditions)
- Aldehydes can be reduced to primary alcohols, and ketones to secondary alchols, using reducing agents such as NaBH4.
(Reverse reaction of primary alcohols being oxidised to aldehydes and carboxylic acids, secondary to ketones) - Aqueous NaBH4 (sodium tetrahydridoborate(III)) is the reducing agent; in each of the negative ions (Na+ and BH4-), one of the bonds is a coordinate (dative covalent) bond using the lone pair on a hydride ion (H-) to form a bond with an empty orbital on the aluminium or boron.
- H4 = fuck tonne of hydrogens = reducing.
- The H- ions come from here (NaBH4 generates it); acts as nucleophile, attaches to Cδ+.
- *Nucleophilic addition**.
- The H+ ions usually come from water; sometimes a weak acid is used as a source of H+.
- 2[H] (the H- [from NaBH4] and the H+ [from solution often weak acid] involved) is often used using reductant in equations, meaning “hydrogen from a reducing agent”.
- H- attacks Cδ+, = O goes to O
- O:- picks up a H+ from solution
- Alcohol fromed.
What occurs when hydrogen cyanide (HCN) reacts with carbonyl compounds?
(mechanism)
HCN + Propanal
- They undergo a nucleophilic addition (nucleophile attacks the molecule, causing an extra group to be added) reaction to produce hydroxynitriles (CN and an OH group present).
- HCN is a weak acid, partially dissociating in water to form H+ and CN- ions.
- The CN- group attacks the Cδ+ atom, donating a pair of electrons. Both electrons from the double bond transfer to oxygen.
- H+ (from other HCN or H2O) bonds to the oxygen to form the hydroxyl group, OH.
- Important reaction in organic synthesis as it increases the length of the carbon chain by one carbon.
- Will produce a racemic mixture of two optical isomers (enantiomers; stereoisomerism present) when carried out with an aldehyde or an unsymmetrical ketone; :CN- ion may attack from above or below the flat C=O plane.
= 2-hydroxybutanenitrile
What occurs when carboxylic acids are added to carbonates?
Butanoic acid + Na2CO3
- They react with carbonates to form a salt, carbon dioxide and water; despite only being a weak acid.
- Carboxylic acids weakly dissociate; the hydrogen of the -OH group is lost forming a stable carboxylate ion; the negative charge shared over it; delocalisation making it stable.
- They are still strong enough to react with sodium hydrogencarbonate, NaHCO3, releasing CO2.
- This distinguishes it from other organic compounds that contain the -OH group.
2[Na+CH3CH2CH2COO- ] + H2O + CO2
What occurs when carboxylic acids are mixed with alcohols?
E.g. ethanoic acid + ethanol?
- Carboxylic acids react with alcohols to form esters, in an esterification reaction.
- THe reaction is speeded up by a strong acid catalyst (usually H2SO4)
- It is also a condensation reaction, releasing water.
- Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind; alkyl.
- Named after the alkyl group first from the alcohol, then the -oic acid group from the carboxylic acid, swapping for -oate.
- Ethyl ethanoate; CH3CO_OCH2CH3_.
(underlined bit is the ethyl group, former bit is the ethanoate) - The esterification reaction is both slow and reversible.
How can you tell an ester has been formed?
(Do esters smell?)
Esters have pleasant smells; as the esters get bigger, the smells tend towards artificial fruit flavouring - “pear drops”, for example.
What are some of the uses of esters?
- Esters have a sweet smell; varying from ‘gluey sweet’ for smaller esters to fruity ‘pear drop’ smells for the larger ones; useful in perfumes.
- Food industry uses esters as food flavourings in drinks and sweets etc.
- Esters are used as solvents; they are polar liquids so lots of polar organic compounds will dissolve in them. They have quite low boiling points; so they evaporate easily from mixtures; good solvents in glues and printing inks.
- Esters are used as plasticisers; added to plastics during polymersiation to make the plastic more flexible.
Over time though, the plasticiser molecules escape, and the plastic becomes brittle and stiff.
What products may the alcohol, propane-1,2,3-triol (glycerol) yield?
- Fatty acids (3x long carboxylic acid chains) combine with glycerol (propane-1,2,3-triol) to make animal fats and vegetable oils (triglycerides); esters of the former.
- The fatty acids can be saturated (no C=C bonds) or unsaturated (with C=C bonds).
- Fats (solid at room temperature) have mainly saturated hydrocarbon chains; fitting neatly together, increasing the van der Waals forces between them.
- Oils (liquid at room temperature) have unsaturated hydrocarbon cahins; the double bonds mean the chains are bent and don’t pack together well, decreasing the effect of van der Waals forces, hence they’re easier to melt and are liquid.
What reaction can vegetables oils and animal fats go through to yield new useful products?
- Esters can be hydrolysed (‘desterification’), with vegetable oils and animal fats yielding soap, glycerol and long cahin carboxylic (fatty) acids as products.
- Hydrolysed by boiliing with NaOH. Both products; glycerol and a mixture of sodium salts of the three carboxylic acids which formed part of the ester, are useful; the salts are soaps (can be a mixture containing many different salts).
The type of soap depends on the fatty acids initially present in the ester. - The sodium salts are ioninc and dissociate to form Na+ and RCOO-; RCOO- has two distinct ends: a long hydrocarbon chain which is non-polar and the COO- group which is polar and ionic; the hydrocarbon mixes with grease, whilst the COO- mixes with water.
- Glycerol readily forms hydrogen bonds and is very soluble in water, due to 3x O-H bonds.
-Used extensive in many pharmaceutical and cosmetic preparations; attracts water, prevents ointments and cream drying out.
-Used as a solvent in many medicines, is present in toothpaste.
-Used as a solvent in the food industry, e.g. for food colourings.
-Used to plasticise shit.
What is biodiesel composed of, and how is it formulated?
- Vegetable oils (e.g. rapeseed oil) make good vehicle fules, but you can’t burn them directly in engines; they must be converted to biodiesel first; reacting them with methanol with a KOH catalyst.
- The product is a mixture of methyl esters of long chain carboxylic (fatty) acids.
triester + methanol → glycerol + methylester
- Biodiesel isn’t 100% carbon netural; energy is used to make the fertilizer to grow the grops, in planting, harvesting, converting the oil, and transport; if this energy comes from fossil fuels, the net carbon emissions to the atmosphere till not be zero.
What is an acyl chloride?
What is its mechanism of reaction?
- Acyl chlorides have the functional group -COcl, with the general formula CnH2n-1OCl.
- All their names end in -oyl chloride. (e.g. ethanoyl chloride)
- The carbon atoms are numbered from the end with the acyl functional group (same as carboxylic acids).
- Acyl chlorides undergo nucleophilic addiion-elimination reactions with water, alcohols, ammonia and primary amines.
(H2O:, HO:CH3, :NH3, CH3N:H2
- The nucleophile attaches onto the Cδ+, with the pair of electrons from the C=O bond going to the O, making it O-: (addition)
- The electron pair from O-: refroms the double bond and the chlorine fucks off.
- The chlorine bonds with the hydrogen of the hydroxyl group, with the electrons from the O-H bond going to O+.
- HCl is eliminated. (elimination)
- The hydrogen atom of the nucleophile has been replaced with an acyl group.
How readily the reaction occurs depends on:
- The magnitude of the δ+ charge pm tje carbonyl carbon, which in turn depends on the elctron-releasing/attractive power of Cl.
- How easily Cl is lost.
- How strong the nucleophile is.
Ammonia is most reactive, then primary amines, then alcohols, then water.
What is the reaction of ethanoyl chloride + water?
(mechanism)
- Hydrolysis reaction (addition of a water molecule to break bonds), of nucleophilic addition-elimination.
- It is a vigorous reaction, producing a carboxylic acid.
- Ethanoic acid is formed.
(with steamy HCl gas side product)
CH3COCl + H2O → CH3COOH + HCl
What is the reaction of ethanoyl chloride + ethanol?
(mechanism)
- The ester, ethyl ethanoate is produced (with steamy HCl fumes)
- Nucleophilic addition-elimination
- A vigorous reaction at room temperature, producing an ester.
- Easy way of producing an ester from an alcohol because it happens at room temperature, and is irreversible.
→ Making an ester from an alcohol and a carboxylic acid (the usual alternative method) needs heat, a catalyst and is reversible - so that it is difficult to get a 100% conversion.
CH3COCl + CH3CH2OH → CH3COOCH2CH3 + HCl
What is the reaction of ethanoyl chloride + ammonia?
(mechanism)
- An amide is produced; ethanamide.
- The H+ ion that is lost then reacts with a second molecule of ammonia to form NH4+; a second NH3 removes a proton from the intermediate ion to give the final product.
- :Cl- then binds with the NH4+ ammonium ion.
What is the reaction of ethanoyl chloride + methylamine?
(mechanism)
- An N-substituted amide is produced, with HCl.
- The product is N-methylethanamide.
- The amine can be regenerated by reaction with NaOH.
What are the advantages of using ethanoic anhydride as an acylating agent than ehtanoyl chloride in the production of aspirin?
- Cheaper
- Less corrosive
- Less easily hydrated; does not react with water as readily, slower
- No toxic HCl fumes produced; by-product is ethanoic acid instead
- Less exothermic
