3.5 Alcohols Flashcards
What are some properties of alcohols?
They are a homologous series Functional group OH General formula: Cn H2n+1 OH The prefix hydroxy is used if another functional group is present V-shaped on the end High boiling points
Why do alcohols have high boiling points?
The have hydrogen bonding
What are the types of alcohol?
The carbon with the OH functional group has
Primary - only 1 other carbon attached
Secondary - 2 other carbons attached
Tertiary - 3 other carbons attached
What are some uses of alcohols?
Perfumes/ aftershave
Mouthwash
Used in manufacture of drugs, detergents and inks
Does ethanol dissolve in water? Why?
Yes
They both have hydrogen bonding
How can we produce alcohols?
Fermentation
Hydration of alkenes
What is involved in fermentation of alcohols?
Yeast, sugar and water is heated between 30-40 degrees (37 optimum)
After several days the flask will contain around 15% ethanol = mixture of alcohol and water
As yeast is living this is a natural process - anaerobic respiration takes place
What is the equation for fermentation?
Glucose ——> ethanol + CO2
Over the arrow: enzymes in yeast
Under the arrow: 30-40 degrees
What is produced in fermentation?
A mixture of ethanol and water
The only product - alcoholic drinks
Why is the temperature conditions as they are for fermentation?
Too low = prevents respiration of the yeast
Too high = denatures the enzymes
What is used to acquire the pure alcohol from fermentation - in the lab?
Fractional distillation
Alcohol boils at 80 degrees
Water boils at 100 degrees
What are some advantages and disadvantages for fermentation?
Renewable sugars used
Aqueous solution of ethanol - could be good if small quantities are needed
Batch process - labour intensive requiring people to change batches
Requires a lot of energy for fractional distillation
How does hydration of alkenes take place?
Ethene is reacted with water(steam)
Using phosphoric acid as a catalyst - H3PO4
Requires high temperature and pressure
Done on a large industrial scale that the majority of alcohols are made from
What is the mechanism for hydration of alkenes?
First arrow from the double bond of the alkene to the H+ ion
A carbocation is formed - second arrow from the lone pair of the oxygen in water to the positive carbon
The water is now attached to the chain with a + charge on the oxygen - the arrow goes from the middle of the O-H on to the oxygen
The alcohol and H+ ion are the final products
What are some advantages and disadvantages for hydration?
Non renewable as the alkenes are from crude oil
Fast continuous process
Pure product - better for larger quantities
Where did the H+ ion come from in hydration and dehydration? Significance?
The phosphoric acid
It takes part in the reaction but is still considered a catalyst as it is regenerated at the end
What is carbon neutral?
No overall carbon dioxide emissions
What is an example of carbon neutral?
Plants absorbing CO2 via photosynthesis
Fermented into ethanol (5-10%) used in biofuels in cars - as it burns nicely with less CO2 produced
Combustion - when the biofuel is used in the cars producing little CO2
The CO2 taken in balances that which is given out
What is dehydration of alcohols?
Turns the alcohol back into an alkene by removing water from it
How does dehydration take place?
Ceramic wool is soaked in an alcohol and heated with aluminium oxide granules
OR
Using phosphoric acid as a catalyst (not the aluminium oxide)
What is the mechanism for dehydration?
First arrow from the lone pair on the oxygen in the alcohol to the H+ ion
On intermediate where the H is attached to the alcohol (like water attached) - first arrow from the middle of the C-O to the oxygen
Then from a the middle of a C-H bond to the middle of the C-C bond
Forming an alkene, water and a H+ ion
What is the simplest oxidation of alcohols?
Combustion producing CO2 + H2O
What is the usual oxidising agent? Written in equations as?
Acidified Potassium dichromate (VI)
- often acidified with dilute sulphuric acid
[O] - you use this as oxygen when balancing
What is the colour change in oxidation of alcohols?
Orange to green
Dichromate (VI) ions are orange
Chromate (III) ions are green
How do we define oxidation in organic chemistry?
Gaining oxygen or losing hydrogen
How do we define reduction in organic chemistry?
Losing oxygen or gaining hydrogen
What does a primary alcohol oxidise to? Using?
A primary alcohol —> aldehyde
Using simple distillation
Because we don’t want it to oxidise further to a carboxylic acid
It works well as aldehyde’s have low boiling points = 20.2 (this is due to no hydrogen bonding)
What is the second stage of oxidising primary alcohols? Using?
Alcohol —> carboxylic acid
Using reflux
So the aldehyde can’t escape and will turn into a carboxylic acid
What does a secondary alcohol oxidise to? Using?
Alcohol —> ketone
Using reflux and acidified Potassium dichromate
What does a tertiary alcohol oxidise to? Why?
No reaction takes place
Can’t oxidise under these conditions
What other oxidising agent could be used?
Acidified Potassium manganate (VII)
What are the potential tests for aldehydes?
Tollen’s test
Fehling’s test
What is Tollen’s test?
Warm the aldehyde with Tollen’s reagent (silver nitrate solution)
A silver mirror will form on the inside of the tube
If a ketone is tested there will be no visible change
What is Fehling’s test?
Warm the aldehyde with Fehling’s solution and a brick red precipitate forms
If a ketone is tested there will be no visible change - remain deep blue
What does Fehling’s solution contain?
A copper complex that is a oxidising agent
Made from Fehling’s A and B
Also called Benedict’s reagent - copper sulfate
What is significant about Tollen’s and Fehling’s reagent?
The will not oxidise an alcohol to an aldehyde or ketone as they are not powerful enough oxidising agents
They only oxidise aldehydes
What are the colours and changes of Tollen’s and Fehling’s for aldehydes?
Tollen’s - clear to silver mirror
Fehling’s - deep blue to brick red
What both contain the carbonyl group?
Aldehydes
Ketones
Ie
C=O
What are the properties of the carbonyl compound in aldehydes?
Polar bond
Always at the end of the carbon chain
No numbers are needed to locate the functional group
FG written as: RCHO
What are the properties of the carbonyl compound in ketones?
In the body of the chain
Locating numbers are needed when 5 carbons are reached
Propanone = simplest ketone
FG written as: RCOR
