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Organic Chemistry > Alcohols 1 > Flashcards

Alcohols 1 Flashcards

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1
Q

What is the general formula of an alcohol

A

CnH2n+1OH or ROH

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2
Q

Describe and explain the molecular ‘shape’ of alcohols

A
  • The oxygen atom has two bonding pairs and two lone pairs
  • The C-O-H angle is about 105 degrees because the 109.5 angle of the tetrahedron is ‘squeezed down’ by the presence of lone pairs.
  • These two lone pairs will repel each other more than the pairs of electrons in a covalent bond.
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3
Q

What is a primary alcohol

A

An alcohol where the carbon that contains the OH functional group only has one R group attached to it. Therefore, the carbon has an OH group, the R group and two hydrogen atoms attached.

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4
Q

What is a secondary alcohol

A

A secondary alcohol is an alcohol where the carbon containing the OH functional group has two R groups attached to it. The carbon therefore has the OH group, two R groups and one hydrogen attached.

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5
Q

What are tertiary alcohols

A

Tertiary alcohols have three R groups attached to the carbon atom that the OH functional group is attached to. This carbon therefore has no hydrogen atoms attached to it.

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6
Q

Why do alcohols have higher boiling points than alkanes with a similiar relative molecular mass

A

The -OH group means that hydrogen bonding occurs between the molecules.

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7
Q

Explain why short chain alcohols are soluble in water but long chain alcohols are insoluble in water

A

The -OH group in alcohols can form hydrogen bonds with water molecules but the non-polar hydrocarbon chain cannot.
This means that the alcohols with short hydrocarbon chains are soluble because the hydrogen bonding predominates. In long chain alcohols the non-polar hydrocarbon chain dominates and this makes these alcohols insoluble in water.

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8
Q

Describe the solubility of alcohols in water

A

Short chain alcohols are soluble in water but long chain alcohols are not.

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9
Q

What are the two methods of producing ethanol

A

Fermentation
Hydration of ethene

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10
Q

What is the equation for the production of ethanol by fermentation from glucose

A

C6H12O6 —> 2C2H5OH + 2CO2

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11
Q

What is the equation for the production of ethanol by the hydration of ethene (reaction of ethene with steam)

A

C2H4 + H2O —> C2H5OH

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12
Q

What are the conditions needed for the production of ethanol via fermentation of glucose

A

Temperature= 35 degrees Celsius
Yeast present

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13
Q

What are the conditions needed for the production of ethanol via hydration of ethene

A
  • Temperature= High temp of 300 degrees Celsius
  • Pressure= 65 atmospheres
  • Catalyst= Phosphoric acid H3PO4
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14
Q

Compare the raw materials needed for the production of ethanol via fermentation vs hydration of ethene

A

The raw material needed for fermentation is sugar (glucose) which is renewable whereas the raw material for hydration of ethene is oil which is non-renewable.

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15
Q

Compare the type of process in the production of ethanol via fermentation vs hydration of ethene

A

Fermentation is a batch process and is ‘carbon neutral’ whereas the hydration of ethene is a continuous process.

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16
Q

Compare the reaction rate/workers needed in the production of ethanol via fermentation vs hydration of ethene

A

The reaction rate of fermentation is slow and many workers are needed whereas the reaction rate of the hydration of ethene is fast and few workers are needed.

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17
Q

Compare the product made when obtaining ethanol via fermentation vs hydration of ethene

A

Fermentation produced impure ethanol (15%) whereas the hydration of ethene produces pure ethanol.

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18
Q

Why is air kept out of the fermentation vessels when producing ethanol

A

To prevent the ethanol oxidising to ethanolic acid.

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19
Q

Why does fermentation produce an impure ethanol solution of 15%

A

Because once the solution contains 15% ethanol, the enzymes in yeast can no longer function and fermentation stops. The ethanol can be distilled from the solution via fractional distillation.

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20
Q

What is the equation for the complete combustion of ethanol

A

C2H5OH + 3O2 —> 2CO2 +3H2O

21
Q

Why are the elimination reaction in alcohols always dehydrations

A

Because a molecule of water is always lost.

22
Q

By which two methods can alcohols be dehydrated

A
  • By being passed over hot aluminium oxide
  • dehydrated using excess of a hot concentrated acid catalyst (sulphuric acid or phosphoric acid)
23
Q

What happens when alcohols are passed over hot aluminium oxide

A

A molecule of water is lost. There is no mechanism- just drawn out formulas.

24
Q

Describe what happens when alcohols are dehydrated using a concentrated acid catalyst

A
  • the lone pair of electrons on the oxygen atom are attracted to the positive hydrogen atom from the acid catalyst.
  • The lone pair forms a covalent bond to the hydrogen ion.
  • An intermediate molecule is formed with a positive oxygen atom.
  • The pair of electrons in the covalent bond between the carbon and oxygen atom move onto the oxygen atom which causes a molecule of water to be released.
  • At the same time, a pair of electrons between a carbon and hydrogen now move between the two carbon atoms. The hydrogen is released as a hydrogen ion and a double bond is formed between the two carbon atoms.
25
Q

What is produced in the dehydration of an alcohol

A

The alkene product and a water molecule.

26
Q

What can primary alcohols be oxidised to

A

Primary alcohols can be oxides to aldehydes which can be oxidised further to carboxylic acids. They can also be oxidised to carboxylic acids straight away

27
Q

What is the general formula of a aldehyde

A

RCHO

28
Q

What is the general formula of a carboxylic acid

A

RCOOH

29
Q

What is the general formulas of a ketone

A

R2CO

30
Q

What can secondary alcohols be oxidised to

A

Ketones. Ketones cannot be oxidised any further.

31
Q

Why can’t tertiary alcohols and ketones be oxidised

A

Oxidation of these compounds would require a C—C bond to break instead of a C—H bond.

32
Q

Describe the experimental details when oxidising alcohols to aldehydes and ketones.

A
  • A solution of potassium dichromate, acidified with dilute sulphuric acid is used as the oxidising agent.
  • In the reaction the orange dichromate (VI) ions are reduced to chromium (III) ions.
33
Q

What conditions are needed to oxidise a primary alcohol into an aldehyde

A
  • heat the alcohol in an oxidising solution of potassium dichromate and dilute sulphuric acid.
  • Distil off the product to prevent the aldehyde further oxidising into a ketone.
34
Q

What conditions are needed to further oxidise an aldehyde into a carboxylic acid

A
  • Mix the aldehyde into a solution of sulphuric acid and potassium dichromate
  • either heat or reflux this solution
35
Q

What conditions are needed to oxidise a primary alcohol straight into a carboxylic acid

A
  • Use a solution of concentrated sulphuric acid and excess potassium dichromate.
  • Reflux the mixture
36
Q

Compare the amount of oxidising agent used to oxidise a primary alcohol into an aldehyde vs a carboxylic acid

A

Twice as much oxidising agent is used to oxidise the alcohol into a carboxylic acid as is used to oxidise it into an aldehyde.

37
Q

What are the conditions needed to oxidise a secondary alcohol into a ketone

A

Acidified potassium dichromate either heated or refluxed.

38
Q

Which property allows us to test wether something is an aldehyde or ketone

A

Aldehydes are affected by gentle oxidation and can be oxidised into a carboxylic acid. Ketones are unaffected by gentle oxidation.

39
Q

What are the two tests that can tell aldehydes and ketones apart

A
  • The Tollens’ (silver mirror) test
  • The Fehling’s test
40
Q

Describe the Tollens’ (silver mirror) test

A
  • Tollens reagent is a solution of silver nitrate in aqueous ammonia and is a gentle oxidising agent.
  • It oxidises aldehydes but has no affect on ketones
  • It contains colourless silver (I) complex ions which are reduced to metallic silver as the aldehyde is oxidised.
  • When the aldehyde is warmed with Tollens reagent, a deposit of metallic silver is formed on the inside of the test tube.
41
Q

Describe Fehlings test

A
  • Fehlings reagent is a gentle oxidising agent.
  • it contains blue copper (II) complex ions that oxidise aldehydes but not ketones.
  • During the oxidation, the blue solution gradually changes to a brick red precipitate of copper (I) oxide.
  • On warming an aldehyde with blue Fehlings solution, a brick red precipitate gradually forms.
42
Q

What do we call an alcohol with two hydroxyl groups

A

A diol

43
Q

What does the volatility of a molecule tell us

A

How readily it turns into a gas

44
Q

Compare the volatility of an alcohol and an alkane with the same number of carbon atoms

A

Alcohols have higher boiling points than alkanes which means that they are less volatile.

45
Q

Explain why, as the nu,her of carbon atoms (in both alcohol and alkane increases), the difference in boiling points between the alcohol and alkane decreases.

A
  • In alkanes, only Van Der Waals act between molecules.
  • In alcohols, both Van Der Waals and hydrogen bonding is present.
  • In an alcohol with a short carbon chain, the major intermolecular force is the hydrogen bonding between hydroxyl groups.
  • In a short chain alkane, Van Der Waals play a much less significant role.
  • This means short chain alcohols have a much higher boiling point than the corresponding alkane.
  • However, in long chain alcohols, the contribution of Van Der Waals forces increases and the relative importance of hydrogen bonding is reduced.
  • This means the boiling points of long chain alcohols are only slightly greater than the corresponding alkanes.
46
Q

Compare the key physical and chemical properties of alkanes and alcohols

A
  • Van Der Waals forces are present in both alkanes and alcohols.
  • hydrogen bonding is present in alcohols, but not alkanes.
  • Alcohols are polar molecules, whereas alkanes are not.
  • Alcohols have higher melting and boiling points than alkanes.
  • Alcohols are less volatile than alkanes.
  • Alcohols are soluble in water whereas alkanes are not.
47
Q

What is the molecular formula of acidified potassium dichromate

A

K2Cr2o7/H+

48
Q

What happens to the oxidising agent acidified potassium dichromate when it is used to oxidise something.

A
  • The dichromate (VI) ion which is orange is reduced to the chromium (III) ion which is green.

Organic Chemistry (14 decks)

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