Alcohols and their different classes

Alcohols

The functional group in alcohols is the hydroxyl group, C–OH.
Alcohols can be classified as primary, secondary or tertiary, depending on how many alkyl groups are bonded to C–OH. You can see how these types of alcohols are named in the diagrams below.

Polarity of alcohols
The properties of alcohols are dominated by the hydroxyl group, C–OH.
Carbon, oxygen and hydrogen have different electronegativity, and alcohols have polar molecules:

The polarity produces electron-deficient carbon and hydrogen atoms, indicated in the diagram above. Depending upon the reagents used, alcohols can react by breaking either the C–O or O–H bond.

Physical properties of alcohols
The –OH group dominates the physical properties of short-chain alcohols. Hydrogen bonding takes place between alcohol molecules, resulting in:

· higher melting and boiling points than alkanes of comparable M,r

· solubility in water.

The solubility of alcohols in water decreases with increasing carbon chain length as the non-polar contribution to the molecule becomes more important.


 

Preparation of ethanol
Ethanol finds widespread uses: in alcoholic drinks, as a solvent in the form of methylated spirits, and as a fuel. There are two main methods for its production.
 

Fermentation of sugars (for alcoholic drinks)

 

Hydration of ethene (for industrial alcohol)
Ethene and steam are passed over a phosphoric acid catalyst at 330°C under high pressure

The preparation method used for ethanol may depend on the raw materials available. An oil-rich country may use oil to produce ethene, then ethanol. However, this method uses up finite oil reserves. In a country without oil reserves (especially in hot climates), sugar may provide a renewable source for ethanol production.

Combustion of alcohols
Alcohols such as ethanol and methanol are used as fuels, making use of combustion.

Ethanol is used as a petrol substitute in countries with limited oil reserves.
Methanol is used as a petrol additive in the UK to improve combustion of petrol.
It also has increasing importance as a feedstock in the production of organic chemicals.

• How do you name aldehydes? How do you name ketones? How do you name substituted aldehydes or ketones?
• Aldehydes and ketones are a group of compounds containing the carbonyl group, C=O.
• Aldehydes always have a hydrogen atom attached to the carbon of the carbonyl group, so the functional group is -CHO (see diagram above).
• The functional group is shown by using 'al' in the suffix part of the name e.g. methanal, ethanal, propanal etc.
• The prefix for the aldehyde name is based on the parent alkane minus the e.
• No number is required for the aldehyde group because the aldehyde group cannot be anything else except carbon atom 1.
•  A number would only be required if a higher ranking group is present e.g. a -COOH carboxylic acid group.
• Ketones always have two carbon atoms attached to the carbon atom of the carbonyl group, so the functional group isC-CO-C (see diagram above).
• The prefix for the ketone name is based on the parent alkane minus the e.
• The functional group is shown by using 'one' in the suffix part of the name e.g. propanone, butanone, hexan-3-one etc.
• A number to denote the position of the ketone group is definitely required beyond butanone and although not strictly needed for butan-2-one, since only one ketone position is possible, it is required for substituted butanones and beyond.
• The substituent numbers are based on giving the carbonyl C=O carbon the lowest number e.g. 2-methylbutanal ('al' position = 1). The number position of the C=O group in ketones needs to be specified for carbon chains of over 4, or less, if substituents present e.g. 3-methylbutan-2-one, heptan-2-one, heptan-3-one and heptan-4-one (there is no heptan-1-one, this is heptanal!).
• For the same 'carbon number', aldehydes and ketones are structural and functional group isomers based on e.g. for aliphatic carbonyl compounds C_nH_2nO.
• Some 'old' names are quoted in (italics) though their use should be avoided if possible [but many still used - just put one into GOOGLE!].

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5.1.2 Examples of Aldehydes

1. methanal (formaldehyde),  ,  , 

Methanal is a trigonal planar shape and the planarity gives H-C=O and H-C-H bond angles of 120^o.

2. ethanal (acetaldehyde),  ,  ,  , 

Bond angles from left to right: H-C-C 109^o, C-C=O 120^o, C-C-H 120^o and O=C-H 120^o.

3. propanal (propionaldehyde),  ,  ,  , 
4. 2-methylpropanal (2 not strictly needed but advisable, 2-methypropionaldehyde),
• ,  , 
5. butanal (butyraldehyde),  ,  , 
6. 2-methylbutanal (2-methylbutyraldehyde),  , 
7. 3-methylbutanal (3-methylbutyraldehyde),  , 
8. pentanal (old name 'valeraldehyde'),  , 
9.  benzaldehyde (benzenecarbaldehyde), and   2-hydroxybenzaldehyde

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5.1.3 Examples of ketones

1. propanone (acetone, dimethyl ketone, DMK),  ,  ,  , 

Bond angles: The trigonal planarity around the carbonyl group >C=O gives a C-C=O and C-C(=O)-C of 120^o and associated with the methyl group, the H-C-C and H-C-H bond angles are 109^o.

2. butanone (or butan-2-one, but 2 not strictly needed, methyl ethyl ketone, MEK, 2-butanone),
• ,  ,  , 
• Bond angles: H-C-C, H-C-H and (O=)C-C-C on right are all 109^o and C-C=O on left, C-C(=O)-C and O=C-C on right are 120^o and the angles associated with the methyl group on the right are all 109^o.
3. 3-methylbutan-2-one (2 and 3 not strictly needed BUT advisable, 3-methyl-2-butanone),
• , 
4. pentan-2-one (2-pentanone),  ,  , 
5. pentan-3-one (3-pentanone),  ,  , 
6. 1-phenylethanone 
7. 2-hydroxyphenylethanone

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5.1.4 Other examples of substituted ketones e.g. iodoketones and phenylketones

1 to 3, 5 to 6 and 10 are formed by the reaction of the parent ketone with iodine in the presence of an acid (e.g. HCl_(aq), because H⁺_(aq) catalyses the reaction)

1.  iodopropanone (1-iodopropanone, 1- not really needed)
2. 1-iodobutanone (1-iodobutan-2-one, -2- not really needed, 1-iodo-2-butanone)
3. 3-iodobutanone (3-iodobutan-2-one, -2- not really needed, 3-iodo-2-butanone)
4. 4-iodobutanone (4-iodobutan-2-one, -2- not really needed, 4-iodo-2-butanone)
5. 1-iodopentan-2-one (-2- needed, 1-iodo-2-pentanone)
6. 3-iodopentan-2-one (-2- needed, 3-iodo-2-pentanone)
7. 4-iodopentan-2-one (-2- needed, 4-iodo-2-pentanone)
8. 5-iodopentan-2-one (-2- needed, 5-iodo-2-pentanone)
9. 1-iodopentan-3-one  (-3- needed, 1-iodo-3-pentanone)
10. 2-iodopentan-3-one (-3- needed, 2-iodo-3-pentanone)
11. Phenyl ketones e.g.
• C₆H₅CH₃COCH₂CH₃  is 1-phenylbutan-2-one (1-phenyl-2-butanone)
• CH₃COCH(C₆H₅)CH₃  is 3-phenylbutan-2-one (3-phenyl-2-butanone)
• CH₃COCH₂CH₂C₆H₅  is 4-phenylbutan-2-one (4-phenyl-2-butanone)

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Alcohol oxidation sequences

Alcohols can be readily oxidised to aldehydes and ketones and aldehydes are easily oxidised further to carboxylic acids.

The reagent can be potassium dichromate(VI) K₂Cr₂O₇ , acidified with diluted sulphuric. acid H₂SO_4(aq) (colour change is orange to green).

However the oxidation products depend on the original structure of the alcohol. The alcohol functional group -OH in aliphatic alcohols is classified into primary, secondary and tertiary types (see below). When the -OH is attached directly to a benzene ring the molecule is called a phenol.

Primary aliphatic alcohols R-OH, R is H or alkyl: When oxidised they form aldehydes and then further oxidation gives a relatively stable carboxylic acid e.g.

1. ==> ==>
• ethanol ==> ethanal ==> ethanoic acid
2. ==> ==>
• 2-methylpropan-1-ol ==> 2-methylpropanal ==> 2-methylpropanoic acid
3.  ==>  ==>
• butan-1-ol ==> butanal ==> butanoic acid
4. ==> ==>
• pentan-1-ol ==> pentanal ==> pentanoic acid

Secondary aliphatic alcohols R-CH(OH)-R', R or R' are both alkyl (can be aryl): When oxidised they form relatively stable ketones (see NOTE below) e.g.

1.  ==>  , propan-2-ol ==> propanone
2.  ==>  , butan-2-ol ==> butanone (butan-2-one)
3.  ==>  , pentan-3-ol ==> pentan-3-one

Tertiary aliphatic alcohols RR'R"C-OH, where R,R' or R" are all alkyl (or aryl): These are relatively stable to oxidation (see NOTE 1. further down) e.g.

1. 2-methylpropan-2-ol ,   or 
2. 2-methylbutan-2-o l,   or 
3. 3-methylpentan-3-o l,   or 

NOTE:

1. Ketones and carboxylic acids are relatively stable to further oxidation because a strong C-C bond must be broken in the process. Prolonged oxidation with H₂SO_4(aq)/K₂Cr₂O₇ or using a more powerful oxidising agent, results in the formation of carbon dioxide, water and carboxylic acids of shorter carbon chain length than the original alcohol or ketone.
2. When a primary alcohol is oxidized to an aldehyde, the oxidation to the carboxylic acid is rapid. If the aldehyde formed first is the desired product, it must be immediately distilled off to prevent further oxidation.

For the reaction:

aA(g) + bB(g) + ... cC(g) + dD(g) + ...

The equilibrium constant based on partial pressures is

From the ideal gas law:

P_AeV = n_ART

n_A is the number of moles of A

R is the ideal gas constant = 0.0821 L•atm/mol•K

T the absolute temperature in K

P is the pressure in atm

V the system volume in L

Similar expressions can be written for each gas phase component.

Rearranging gives

but n_A/V is just the molar concentration = [A]_e

Substituting into the expression for K_p (for each gas phase component) gives

Collecting terms gives

The left part of the fraction is K_c, so

K_p = K_c × (RT)^{(c+d+...)–(a+b+...)}

The exponent in RT is the sum of the stoichiometric coefficients for the reactants subtracted from the sum of the stoichiometric coefficients for the products, defined as  n.

K_p = K_c × (RT) ⁿ

Because the derivation goes through the ideal gas law, the proper units for R in this case are L•atm/mol•K (i.e., R = 0.0821 L•atm/mol•K).