Carboxylic Acids and Esters
- Created by: shaha01
- Created on: 05-01-18 12:11
What are Carboxylic Acids?
- Have the functional group -COOH.
- To name, find and name longest C chain, take off the 'e' and add '-oic acid'
- Carboxyl group is always at end of molecule and when naming is more important than other functional groups-so all other functional groups are numbered starting from this C.
Dissociation of Carboxylic Acids
- Carboxylic acids are weak acids- in water partially dissociate into a carboxylate ion and a H+ ion.
- This reaction is reversible but equilibrium lies to left because most molecules do not dissociate.
Reaction with Carbonates
- Carboxylic acids react with carbonates (CO3 2-) or hydrogen carbonates (HCO3-) to form a salt, carbon dioxide water.
- In these reactions, carbon dioxide fizzes out of the solution.
Esterification Reactions
- If you heat a carboxylic acid with an alcohol in the presence of a strong acid catalyst, you get an ester.
- This is called an esterification reaction.
Concentrated sulfuric acid (H2SO4) is usually used as the acid catalyst, but other strong acids such as HCl or H3PO4 can also be used.
Naming Esters
- Esters made from reacting Alcohol with Carboxylic Acid.
- Name of ester is made up of 2 parts- first bit from alcohol, second bit from carbxylic acid.
1. Look at alkyl group that came from the alcohol. This is the first bit of the esters name.
2. Look at the part that came from the carboxylic acid. Swap it's 'oic acid' ending for 'oate' to get the second bit of the name.
3. Put the two parts together.
- Same rules apply even if C chains are branched/if there is a benzene ring. Always number Cs starting from C atons in the C-O-C bond.
Uses of Esters
- Sweet smell- useful in perfumes. Also good in food industry for flavouring in drinks/sweets.
- Esters are polar liquids- so lots of polar organic compounds will dissolve in them. Also have low BPs so evaporate easily from mixtures, so goos solvents in glues and printing inks.
- Used as plasticisers- make plastics more flexible. Over time, plasticiser molecules escape though and make plastic brittle and stiff.
Hydrolysis
- When substance is split by water.
- Using just water is usually v slow- sped up by using acid or alkali.
- 2 types of hydrolysis of esters- acid hydrolysis and base hydrolysis.
Acid Hydrolysis
- Splits ester into acid and alcohol.
- Have to reflux ester with dilute acid (e.g. hydrochloric/sulfuric acid)
- Ester will then split back into carboxylic acid and alcohol it was originally made from.
As these acid hydrolysis rectons are reversible you need to use lots of water to push equilibrium to right so you get lots of product.
Base Hydrolysis
- Reflux ester with dilute alkali e.g. sodium hydroxide.
- OH- ions from base react with ester and you get carboxylate ion and an alcohol.
Fats and Oils
- Animal fats/Vegetable oils are esters of glycerol and fatty acids.
- Fatty acids are long chain carboxylic acids- Combine with glycerol (propane-1,2,3-triol) to make fats and oils.
- Fatty acids can be saturated (no double bonds) or unsaturated (with C=C double bonds).
- Most of a fat/oil is made from fatty acid chains-so it's these that give them many of their properties.
Structure of a Fat/Oil
"Fats" have mainly saturated hydrocarbon chains-fit neatly together, increading the VDW forces between them. This means higher temps are needed to melt them and they are solid at room temp.
"Oils" have unsaturated hydrocarbon chains- double bond means chains are bent and do not pack together well, decreasing effect of VDW forces. So are easier to melt and are liquids at room temp.
Hydrolysis of Fats and Oils
- Like any ester, can hydrolyse fats and oils by heating with sodium hydroxide. This is a type of base hydrolysis.
- OH- ions from sodium hydroxide react with fat/oil to form carboxylate ion and alcohol. The alcohol formed is glycerol (propane 1,2,3-triol) and the carboxylate ions combine with Na+ ions from the sodium hydroxide to form a sodium salt (a soap).
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Can convert sodium salt back into long chain carboxylic acid (fatty acid) by adding an acid e.g. HCl. H+ ions from acid displace Na+ ions in salt to form carboxylic acid, which releases free Na+ ion:
Biodiesel
- Veg oils e.g. rapeseed oil make good vehicle oils but you cant burn them directly in engines.
- Oils must be converted into biodiesel first.
- Involves reacting with methanol, using strong alkali (e.g. potassium hydroxide or sodium hydroxide) as a catalyst.
- Will get a mix of methyl esters of long-chain fatty acids: This is biodiesel.
Is Biodiesel 100% Carbon Neutral?
- Can be thought of as carbon neutral because when crops grow they absorb same amount of CO2 they produce when burned.
- But not that simple- energy is used to make fertiliser to grow crops, and is used in planting, harvesting and converting the oil.
- If this energy comes from fossil fuels, then the process will not be carbon neutral overall.
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