Carbonyl compounds, carboxylic acids and their derivatives

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Naming carbonyl compounds

  • Contain a C=O  functional group.
  • They are either aldehydes or ketones depending on where the C=O is (Aldehyde is on the side, ketone is in the midde zone.

Naming carbonyl compounds

  • Aldehydes are givven the suffix -al. They do not need a locant number to indicate which carbon the =o is attached to because it is always on a terminal carbon.
  • Ketones are give the suffix -one.

If other higher priority functional groups e.g. carboxylic acids are present on the same molecule, then carbonyls and their positions can be indicated using the prefix oxo- .

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The carbonyl group

  • Carbonyls are unsaturated compounds like alkenes (due to the double bond).
  • Nucleophiles (:Nu-) are attracted to the slight positive charge on the carbon atom
  • C=O is more polar than C=C because the difference in electronegativity is higher, so the C has a delta positive charge, due to its electrons being pulled away from it and towards the oxygen, which is now delta negative.

Carbonyl solubility

  • Low molecular mass carbonyls (e.g. short shain ketones and aldehydes) are fairly soluble in water, as they are able to form hydrogen bonds with the water molecules.
  • A hydrogen bond is a strong intermolecular force of attraction between an unsheilded proton (the hydrogen's proton has become exposed as its electrons have been pulled away from it) and a lone pair of electrons on an oxygen, nitrogen or flourine molecule.
  • Solubility decreases. As the molecule gets longer in carbon chain length, a greater proportion of the molecule canot form hydrogen bonds and solubility decreases.
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Oxidation of alcohols diagram

(http://www.chemgapedia.de/vsengine/media/vsc/en/ch/2/oc/reaktionen/formale_systematik/oxidation_reduktion/oxidation/entfernen_wasserstoff/oxidation_alkohole/oxschema_en.gif)

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[O] = Oxidation using oxidants like acidefied K2Cr

The colour change expected when potassium dichromate dissolved in dilute H2SO4 (acidefied potassium dichromate VI) oxidise an alcohol is:

ORANGE --------> GREEN

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Reduction diagram

                      Image result for reduction of alcohols

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Reduction of aldehydes and ketones

NaBH4 and LiAH4 are both very strong reducing agents (reductants). Both of these produce a nucleophile, which is H- (a hydride ion). This ion is a nucleophile because it is an electron donor and is attracted to areas with a positive charge.

E.g. reduction of ethanal

Image result for reduction of ethanal by lialh4 (http://www.docbrown.info/page15/mech40.gif)

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Summary of reactions of ketones vs aldehydes

Reaction                                            Aldehyde                                           Ketone

Acidified K2Cr2O7 (VI)                     Orange -> Green                               No reaction

Fehling's solution                            Red precipitate                                  No reaction

Brady's Reagent                             Red/orange/yellow precip.                 Red/orange/yellow precip.

Tollen's Reagent                            Silver mirror                                        No reaction

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The carboxyl functional group

The carboxyl group contains both the carbonly  C=O group and the alcohol (hydroyxl) O-H group. We may expect some changes in behaviour of these groups if they interact with each other.

Given the nature of the carboxyl group- we can predict how the melting point of a carboxylic acid would compare to that of the corresponding alkane e.g. propanoic acid and propane. We can see that the melting point of the carboxylic acid would be higher than that of the corresponding alkane., because they have hydrogen bonds (between the O and the H), however alkanes just have weak Van Der Waals forces.

Solubility of low mass

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Acid strength of carboxylic acids

When carboxylic acids dissociate to produce H+ ions, the O-H bond is broken

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Carboxylic acid dissociating to produce a proton

Image result for ethanoic acid dissociation (http://www.everythingmaths.co.za/science/grade-12/04-organic-molecules/pspictures/df333b8f85031847a8dfd90087a36654.png)

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Explaining acid strength

The strength of an acid refers to its ability or tendency to lose a proton (H+). A strong acid is one that completely ionizes (dissociates) in a solution (provided there is sufficient solvent).

 An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution.

Ka of ethanoic acid is  1 x 10-5

Ka of ethanol is 1 x 10-16  

Ethanoic acid is a stronger acid than ethanol because more hydrogen ions can dissociate from their molecules than in ethanol. This is due to the delocalisation of the negative charge which inhibits a reverse reaction and doesn't happen as readily.

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Carboxylic acids as acids

Carboxylic acids undergo salt forming reactions in a similar way to inorganic acids like HCl and H2SO4.

Acid + Metal -----> Salt + Hydrogen

E.g. 2HCOOH + Mg ---> (HCOOH)2Mg2+ (magnesium methanoate)  + H2

Acid + Base ( E.g. metal oxides and hydroxides) ------> Salt + Water

E.g. CH3COOH + NaOH ---> CH3COO-Na+ (sodium ethanoate) + H2O

Acid + Carbonate -----> Salt + CO2 + H2O

E.g.  2C6H5COOH + Na2CO3 ---> 2C6H5COO-Na+ +CO2 +H2O

Acid + Alkali -----> Salt + Water

E.g.

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Test for carboxylic acids

  • Add Na2CO3- If the compound is a carboxylic acid, effervescence is seen, as carbon dioxide is evolved.

E.g. 2CH3COOH + Na2CO3 ---> 2CH3COO-Na+ +CO2 +H2O

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Reduction of Carboxylic Acids

Carboxylic acids can be reduced to primary alcohols via aldehydes using reducing reagents like LiAlH4 and NaBH4.

LiAlH4 is used with dry ether

NaBH4 is used with methanol and water

  • Carboxylic acid ----> Aldehyde ----> Primary alcohol
  •                          [H]                    [H]

Both reducting agents (LiAlH4 and NaBH4) provide a :H- nucleophile (an electron donor)

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Esterification

  • The formation of esters from carboxylic acids and alcohols involves a nucleophilic attack by an alcohol on the carbon atom of the carboxyl group.
  • The alcohol (hydroxyl group) can act as a nucleophile because of the lone pairs of electrons on the O atom.
  • These reactions need to be catalysed by concentrated sulfuric acid  and generally need to be refluxed.
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Naming esters

 Image result for naming esters (http://images.slideplayer.com/27/9196841/slides/slide_5.jpg)

The part of the ester molecules with C=O is derived from the carboxylic acid and form the second part of the ester name e.g. methyl propanoate. The part of the ester molecule with O-R is derived from the alcohol and forms the first part of the name e.g methyl propanoate.             

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Uses of esters

  • Esters are used in the manufacture of perfumes and fruit flavouring due to their fruity smells.
  • Solvents
  • Plasticisers
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Esters as plasticisers

                                        Image result for pvc and plasticisers

Let grey = carbon
Let green = chlorine

The C-Cl bond in PVC (Polyvinyl chloride) is polar due to the differences in electronegativity between C and Cl, meaning that carbon has a delta positive charge and chlorine has a delta negative charge. Therefore, intermolecular forces between neighboring PVC chains are permanent dipole-dipole attractions. Due to the size of the PVC chains and the extensive nature of these inter-molecular forces, the PVC chains are held together strongly and so the PVC is a hard, rigid material as the polymer chains don't move very easily.

Ester molecules fit between the PVC chains and push them apart, meaning the chains aren't as strongly attracted to each other, so they help PVC become  more flexible.

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Hydrolysis of esters

These reactions split esters into their 2 component parts. There are two different ways an ester can be hydrolysed, which yield two slightly different products.

1. Acid Hydrolysis - (Dilute HCl/H2SO4 and heat under reflux)
This is the reverse of esterification and so an alcohol and a carboxylic acid are produced
Ester + water ----> Carboylic acid + Alcohol

2. Alkaline Hydrolysis
In this reaction, an alcohol and a salt of the carboxylic acid are made (a carboxylate).
Ester + NaOH ----> ROH + RCOO-Na+ (salt of the acid)

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Saponification of Fats (manufacture of soap)

Fats contain triglycerides- these are esters of the alcohol glycerol (IUPAC name is propane-1,2,3-triol). and long chain carboxylic acids (approximately 20 Carbons) are called fatty acids.

In saponification, alkaline hydrolysis occurs as the triglyceride is boiled with alkaline solution.
This yields the original alcohol (glycerol) and a sodium salt of the long chain fatty acid, which is the active constituent of soap.  

Structure of a soap molecule:

 Image result for structure of soap (http://upload.wikimedia.org/wikipedia/commons/7/73/StericAcidChemStr.png)

The hydrophobic hydrocarbon tail is non polar and does not generally dissolve in water but is much more soluble in grease. The hydrophilic head is an anionically (negatively) charged carboxylate group (COO-) and is soluble in water but mixes less readily with fats/oils like grease.

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Carboxylic derivatives

These are molecules in which the O-H group of the carboxylic acid are replaced with another atom or group.

CH3C=OW:

  • If W is OH, the molecule is a carboxylic acid
  • If W is OR, the molecule is an ester
  • If W is Cl, the molecule is an acyl chloride
  • If W is NH2, the molecule is a amide
  • If W is NHR, the molecule is a nitrile
  • If W is OCOR, the molecule is an acid anhydride
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Acyl chloride and water

Carboxylic derivatives can undergo nucleophilic addition elimination reactions, in which a nucleophile replaces the group W in the derivative (e.g. -OH, -NH2, -OCOR etc.).

                         Image result for ethanoyl chloride and water

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Relative reactitivies of carboxylic acid derivativ

               Image result for reactions of carboxylic acid derivatives

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Acyl chloride with alcohols

Nucleophilic addition-elimination reaction

Acyl chloride + Alcohol ---> Ester + HCl

Room temperature and anhydrous conditions will create a high yield

Image result for acyl chloride and alcohol (http://www.docbrown.info/page15/mech15.gif)

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Acyl chloride with ammonia

Nucleophilic addition-elimination reaction

Acyl chloride + Ammonia ----> Amide + HCl

Room temperature and anhydrous conditions create a high yield

Image result for acyl chloride and alcohol (http://alevelchem.com/img/ammonia%2Bacyl_chloride.gif)

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Acyl chloride with amines

Acyl chloride + Amine ----> N-substituted amide and hydrochloric acid

Look for steamy fumes due to the ammonium chloride produced when HCl reacts with NH3

Anhydrous conditions and room temperature produce a high yield. 

Image result for acyl chloride and amines (http://alevelchem.com/img/ethanoyl_chloride_plus_amine.gif)

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