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alcohols
alcohols
alcohol - further information on hydrogen bonding
alcohol - further information on hydrogen bonding
AFFECTING SOLUBILITY AND BOILING POINT
AFFECTING SOLUBILITY AND BOILING POINT
This is due to the lengthening hydrophobic chains which contain temporarily induced dipoles, to which the water has no affinity.
This is due to the lengthening hydrophobic chains which contain temporarily induced dipoles, to which the water has no affinity.
In effect the when alcohols dissolve hydrogen bonds are broken between water molecules (within solvent forces) and hydrogen bonds between alcohol molecules (within solute forces) are broken; then new hydrogen bonds between the water and alcohol molecules are (solvent - solute forces).
In effect the when alcohols dissolve hydrogen bonds are broken between water molecules (within solvent forces) and hydrogen bonds between alcohol molecules (within solute forces) are broken; then new hydrogen bonds between the water and alcohol molecules are (solvent - solute forces).
This takes energy: 'bonds' are broken (endothermic) and bonds are made (exothermic), with energy needed to do both being equal. With higher mass alcohols the longer hydrocarbon chain (tail) cannot form new hydrogen bonds with water; they contain weak intermolecular forces (Van der Waal's) and do not have enough energy in them to make new bonds with water.
This takes energy: 'bonds' are broken (endothermic) and bonds are made (exothermic), with energy needed to do both being equal. With higher mass alcohols the longer hydrocarbon chain (tail) cannot form new hydrogen bonds with water; they contain weak intermolecular forces (Van der Waal's) and do not have enough energy in them to make new bonds with water.
At this level you do not need to have an in depth understanding of Hydrogen bonding, but here is a basic overview.
The graph below shows the effect of hydrogen bonding on increased boiling point in compounds in groups 14 - 17.
The graph below shows the effect of hydrogen bonding on increased boiling point in compounds in groups 14 - 17.
ALCOHOL REACTIONS - OXIDATION AND REDUCTION
ALCOHOL REACTIONS - OXIDATION AND REDUCTION
PRIMARY ALCOHOLS ARE OXIDISED TO CARBOXYLIC ACIDS, WITH ALDEHYDES BEING AN INTERMEDIATE PRODUCT
PRIMARY ALCOHOLS ARE OXIDISED TO CARBOXYLIC ACIDS, WITH ALDEHYDES BEING AN INTERMEDIATE PRODUCT
SECONDARY ALCOHOLS ARE OXIDISED TO KETONES
SECONDARY ALCOHOLS ARE OXIDISED TO KETONES
TERTIARY ALCOHOLS DO NOT OXIDISE
TERTIARY ALCOHOLS DO NOT OXIDISE
THE REVERSE REACTION, REDUCTION, CAN TURN THE ALDEHYDE OR KETONE BACK INTO THE RESPECTIVE PRIMARY OR SECONDARY ALCOHOL
THE REVERSE REACTION, REDUCTION, CAN TURN THE ALDEHYDE OR KETONE BACK INTO THE RESPECTIVE PRIMARY OR SECONDARY ALCOHOL
alcohols - tests used to distinguish between classes of alcohols
alcohols - tests used to distinguish between classes of alcohols
LUCAS REAGENT TEST
LUCAS REAGENT TEST
TO DISTINGUISH BETWEEN 1°, 2° AND 3° ALCOHOLS
TO DISTINGUISH BETWEEN 1°, 2° AND 3° ALCOHOLS
Lucas Reagent is composed of HCl (conc) / ZnCl₂ (anhydrous).
Lucas Reagent is composed of HCl (conc) / ZnCl₂ (anhydrous).
This is a nucleophilic substition reaction - of a Cl⁻ for an OH⁻ group. The video shows the results.
This is a nucleophilic substition reaction - of a Cl⁻ for an OH⁻ group. The video shows the results.
3° alcohols change almost instantaneously as an insoluble chloroalkane is formed
3° alcohols change almost instantaneously as an insoluble chloroalkane is formed
2° take longer, whilst 1° alcohols take the longest, if at all, Both will need heat
2° take longer, whilst 1° alcohols take the longest, if at all, Both will need heat
tollen's test
tollen's test
This test can be used to distinguish between 1° and 2° alcohols. Tollen's is not a strong enough oxidant to oxidise alcohols but it will oxidise aldehydes to carboxylic acids. Remember aldehydes are an intermediate product in the oxidation of 1° alcohols, when done under distillation. Tollen's cannot oxidise ketones, which are produced when 2° alcohols are oxidised. A positive test with Tollen's indicates that the alcohol was a 1° alcohol, as the aldehyde is oxidised and the Tollen's is reduced.
This test can be used to distinguish between 1° and 2° alcohols. Tollen's is not a strong enough oxidant to oxidise alcohols but it will oxidise aldehydes to carboxylic acids. Remember aldehydes are an intermediate product in the oxidation of 1° alcohols, when done under distillation. Tollen's cannot oxidise ketones, which are produced when 2° alcohols are oxidised. A positive test with Tollen's indicates that the alcohol was a 1° alcohol, as the aldehyde is oxidised and the Tollen's is reduced.
A few drops of Tollen's are added to the sample to be tested, which then needs to be warmed.
A few drops of Tollen's are added to the sample to be tested, which then needs to be warmed.
Tollen's contains the silver diamine ion
Tollen's contains the silver diamine ion
Fehling's / Benedicts Solution Test
Fehling's / Benedicts Solution Test
This is a mild oxidant and cannot oxidise ketones; it oxidises aldehydes, showing the the alcohol was a 1° alcohol.
This is a mild oxidant and cannot oxidise ketones; it oxidises aldehydes, showing the the alcohol was a 1° alcohol.
This video shows this test.
This video shows this test.
Oxidation with Cr₂O₇²⁺/H⁺ or MnO₄⁻/H⁺
Oxidation with Cr₂O₇²⁺/H⁺ or MnO₄⁻/H⁺
Remember MnO₄⁻/H⁺ can be used to oxidise alkenes to diols.
These reactions can be used to distinguish that the alcohol is either a 1° and 2° alcohols and not a 3° alcohol, which cannot be oxidised. To oxidise a 3° alcohol a C - C bond would need to be overcome.
These reactions can be used to distinguish that the alcohol is either a 1° and 2° alcohols and not a 3° alcohol, which cannot be oxidised. To oxidise a 3° alcohol a C - C bond would need to be overcome.
The video below shows oxidation of alcohols using acidified dichromate as a test to distinguish between primary and secondary as opposed to tertiary alcohols.
The video below shows oxidation of alcohols using acidified dichromate as a test to distinguish between primary and secondary as opposed to tertiary alcohols.
This video shows a more vigourous oxidation reaction
This video shows a more vigourous oxidation reaction
aldehydes
aldehydes
and
and
ketones
ketones
For a review of naming and functional groups go here
or scan the QR code,
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Aldehydes and ketones with the same number of carbons are isomers of each other.
Aldehydes and ketones with the same number of carbons are isomers of each other.
The video below from Khan Academy goes through the properties of Aldehydes and Ketones.
The video below from Khan Academy goes through the properties of Aldehydes and Ketones.
aldehydes and ketones - physical properties
aldehydes and ketones - physical properties
Carbonyl group: -C = O
Carbonyl group: -C = O
The C = O bond is polar in nature due to the large difference in electronegativity between O and C; Oxygen is much more electronegative. This means that the O has a strong tendency to pull the electrons shared in this bond towards the Oxygen end of the bond; in fact one of the pairs is even more easily pulled this way. This polarity explains some of the physical properties of these compounds.
The C = O bond is polar in nature due to the large difference in electronegativity between O and C; Oxygen is much more electronegative. This means that the O has a strong tendency to pull the electrons shared in this bond towards the Oxygen end of the bond; in fact one of the pairs is even more easily pulled this way. This polarity explains some of the physical properties of these compounds.
physical properties - Boiling point
physical properties - Boiling point
As can be seen from the graph aldehydes and ketones with the same number of carbons have very similar boiling points (remember they are structural isomers) and they both have considerably higher boiling points than the corresponding alkane. This is a function of the relative size and strengths of the intermolecular forces.
As can be seen from the graph aldehydes and ketones with the same number of carbons have very similar boiling points (remember they are structural isomers) and they both have considerably higher boiling points than the corresponding alkane. This is a function of the relative size and strengths of the intermolecular forces.
The alkanes only have the temporarily induced dipoles whereas adehydes and ketones contain a polar bond, and are polar molecules, having stronger permanent dipoles. It is important to remember here that they cannot hydrogen bond to themselves, as the they do not have a Hydrogen bonded to the oxygen. This accounts for the fact that the boiling points of both are less than the corresponding alcohol, as shown in the table below.
The alkanes only have the temporarily induced dipoles whereas adehydes and ketones contain a polar bond, and are polar molecules, having stronger permanent dipoles. It is important to remember here that they cannot hydrogen bond to themselves, as the they do not have a Hydrogen bonded to the oxygen. This accounts for the fact that the boiling points of both are less than the corresponding alcohol, as shown in the table below.
physical properties - solubility
physical properties - solubility
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