CH0003 - Lecture 3 - Alcohols

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  1. What is an alcohol?
    What are their general formulae?
    Alcohol definition: an alcohol is an organic compound that contains one (or more) hydroxyl group (-OH) that is attached to a saturated carbon atom i.e. a carbon atom that has another three single bonds to other atoms.

    • General formulae
    • Simple saturated mono-alcohols CnH2n+1OH

    Which is why number of oxygen atoms doesn’t need to be taken into account in DBE calculations.
  2. What is a hydroxyl group?
  3. How can alcohols be classified?
    • Alcohol classification
    • Typically classified by the type of carbon to which the hydroxyl group is attached

    Primary (1oCarbon bonded to the OH is attached to one other alkyl group (or none for methanol)

    Secondary (2oCarbon bonded to the OH is attached to two alkyl groups

    Tertiary (3oCarbon bonded to the OH is attached to three alkyl groups
  4. All the alcohols end with the suffix...
    -ol, as they are alcohols
  5. Comment on the physical properties of alcohols.
    Water derivative?

    Sometimes it is useful to view alcohols as derivatives of water with one hydrogen replaced with an alkyl group.

    The hydroxyl functional group is polar (oxygen electronegativity 3.8, Hydrogen 2.1 – strong dipole so very polar)

    The intermolecular forces between any given alcohols are: Hydrogen bonding (cf. water) and VdW (cf. hydrocarbons)
  6. Comment on the solubility of alcohols
    In low molecular mass alcohols the H-bonds predominate and therefore they mix completely with water.

    As the molecular weight increases i.e. the hydrocarbon portion becomes larger, the solubility of the alcohols in water decreases and eventually approaches that of the hydrocarbon.

    The low molecular mass alcohols have much higher boiling points than their comparable hydrocarbons (greater difference) – due to extensive H-bonding.

    As their mass increase the H-bonding effects becomes less (more difficult also) and VdW forces become more significant.
  7. Comment on the boiling points of alcohols.
    The boiling point of an alcohol is always much higher than that of the alkane with the same number of carbon atoms.

    The boiling points of the alcohols increase as the number of carbon atoms increases.

    The patterns in boiling point reflect the patterns in intermolecular attractions.
  8. Comment on hydrogen bonding in alcohols
    Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen.

    In the case of alcohols, there are hydrogen bonds set up between the slightly positive hydrogen atoms and lone pairs on oxygens in other molecules.

    The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms.

    In alkanes, the only intermolecular forces are van der Waals dispersion forces. Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules.

    That's the main reason that the boiling points are higher.
  9. Comment on the effect of van der Waals forces on alcohols
    Hydrogen bonding isn't the only intermolecular force in alcohols. There are also van der Waals dispersion forces and dipole-dipole interactions.

    The hydrogen bonding and the dipole-dipole interactions will be much the same for all the alcohols, but the dispersion forces will increase as the alcohols get bigger.

    These attractions get stronger as the molecules get longer and have more electrons. That increases the sizes of the temporary dipoles that are set up.

    This is why the boiling points increase as the number of carbon atoms in the chains increases. It takes more energy to overcome the dispersion forces, and so the boiling points rise.
  10. How can alcohols be used?
    Alcohol functional group is very useful for organic chemistry –it can be transposed into many other functional groups

    Conversion to haloalkanes (alkanes that contain a halogen e.g. Cl)

    Dehydration to an alkene (‘removal of water’)

    Oxidation to: aldehydes; ketones; and carboxylic acids

    Conversion to esters
  11. What is an alkyl?
    In patent chemistry, an alkyl substituent is an alkane missing one hydrogen.
  12. What are ketones and aldehydes?
    Aldehydes and ketones are simple compounds which contain acarbonyl group - a carbon-oxygen double bond. They are simple in the sense that they don't have other reactive groups like -OH or -Cl attached directly to the carbon atom in the carbonyl group - as you might find, for example, in carboxylic acids containing -COOH.

    In aldehydes, the carbonyl group has a hydrogen atom attached to it together with either  a second hydrogen atom or, more commonly, a hydrocarbon group which might be an alkyl group or one containing a benzene ring.

    In ketones, the carbonyl group has two hydrocarbon groups attached. Again, these can be either alkyl groups or ones containing benzene rings. Notice that ketones never have a hydrogen atom attached to the carbonyl group.
  13. What is carboxylic acid?
    A carboxylic acid is an organic compound that contains a carboxyl group 

    The carboxyl group is an organic functional groupconsisting of a carbon atom double bonded to an oxygen atom andsingle bonded to a hydroxyl group.
  14. What is alcohol halogenation?
    Treatment of an alcohol with specific reagents yields the corresponding halo alkane derivative

    Halogenoalkanes can be made from the reaction between alkenes and hydrogen halides, but they are more commonly made by replacing the -OH group in an alcohol by a halogen atom.

    • The general reaction looks like this:
    • ROH + HX ---> RX + H2O
  15. What are examples of some haloalkanes?
    Fluoroalkane / alkyl flouride - OH swapped for F

    Chloroalkane / alkyl chloride - OH swapped for Cl

    Bromoalkane / alkyl bromide - OH swapped for Br

    Iodoalkane / alkyl iodide - OH swapped for I
  16. What is alcohol oxidation?
    • Treatment of an alcohol with a strong oxidising agent results in the formation of aldehydes, ketones and even carboxylic acids (if vigorous enough)
    • However, not all alcohols can be oxidised
  17. Describe the oxidation of primary alcohols (1o)
    • Primary alcohols can be oxidised to either aldehydes or carboxylic acids depending on the reaction conditions.
    • In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid.
  18. How are primary alcohols oxidised to aldehydes? Formula
    If you used ethanol as a typical primary alcohol, you would produce the aldehyde ethanal, CH3CHO.

    In organic chemistry, simplified versions are often used which concentrate on what is happening to the organic substances. To do that, oxygen from an oxidising agent is represented as [O]. That would produce the much simpler equation:

    3CH3CH2OH + [O] ---> CH3CHO + H20
  19. How are primary alcohols oxidised to aldehydes? Structure picture
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  20. What is the formula for the full oxidation of ethanol to ethanoic acid?
    CH3CH2OH + [O] -> CH3CHO + H20

    CH3CHO + [O] -> CH3COOH
  21. What is the second stage of ethanoic acid being oxidized to ethanoic acid? Structure picture
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  22. How are secondary alcohols oxidised?
    Secondary alcohols are oxidised to ketones - and that's it. For example, if you heat the secondary alcohol propan-2-ol with sodium or potassium dichromate(VI) solution acidified with dilute sulphuric acid, you get propanone formed
  23. How are secondary alcohols oxidised? Picture
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  24. How are tertiary alcohols oxidised?
    Tertiary alcohols aren't oxidised by acidified sodium or potassium dichromate(VI) solution. There is no reaction whatsoever.

    If you look at what is happening with primary and secondary alcohols, you will see that the oxidising agent is removing the hydrogen from the -OH group, and a hydrogen from the carbon atom attached to the -OH. Tertiary alcohols don't have a hydrogen atom attached to that carbon.

    You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond.
  25. How are tertiary alcohols oxidised? Picture
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  26. What is alcohol dehydration?
    Dehydration – removal of water

    The simplest alcohol this can be done on is ethanol

    The dehydration can be achieved with acid catalysis using sulfuric acid (concentrated).
  27. What is the equation for the dehydration of ethanol?
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  28. What are esters?
    • Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind.
    • We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).
  29. What is a common ester? With picture
    The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:

    Image Upload

    Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.
  30. How are esters produced?
    Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid.
  31. What is the formula for esterification?
    The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:

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    So, for example, if you were making ethyl ethanoate from ethanoic acid and ethanol, the equation would be:

    Image Upload
  32. What are esters for?
    Esters are important chemicals, a specific use is in the formation of a particular polymer – polyester

    To form these polymers there must be alcohol functional groups at both ends and the same for the carboxylic acid – a di-ol and a di-carboxylic acid

    Biological importance of esters?

    Fats (fatty acids) in the body are stored as triglycerides (triesters!)

    Saturated / unsaturated fatty acids – link to alkane / alkenes
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CH0003 - Lecture 3 - Alcohols
2014-03-20 13:30:29
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