Organic 1, 3, 4 - Organic Families & Reactions associated

aliphatic hydrocarbon: alkane

1. all members of the alkanes have single bonds around/between carbons; i.e. they are saturated compounds

2. end in -ane

3. general formula is CnH2n+2

4. C1 - C4 = gases; C5 - C16 = liquids; C17+ = waxy solines

5. alkane molecules are held together by weak VdW forces of attraction. BPs increase with increasing chain length as greater surface contact between molecules means greater numbers of VdW forces between the molecules

6. each successive member of the alkanes differs by CH2

7. alkanes are insoluble in water as they are non-polar molecules (tetrahedral shape) and don’t form hydrogen bonds with water, but they are soluble in non-polar solvents such as cyclohexane or chloroform

8. bc they are saturated, they are unreactive; alkanes don’t decolourise bromine water from red/brown to colourless or a dilute solution of KMnO4 from purple to colourless → both of these tests prove that alkanes are saturated compounds

9. when they react, they typically undergo free radical substituion reactions

remember these are named alphabetically & alkyl group (e.g. methy and ethyl), put on the FIRST C of a molecule

reaction of alkanes

free radical substitution reaction: when an atom of group of atoms are replaced by another atom or groups of atoms

• CH4 + Cl2 → CH3Cl + HCl

• reagent = Cl2

• condition = UV light

• HCl = hydrogen chloride (not in aqueous solution)

aliphatic hydrocarbon: alkene

• all members have 1 double bond b/n two of the carbon atoms, therefore unsaturated

• ending is -ene

• C2-C4 = gases; C5-C16 = liquids; C17+ = waxy solids

• held together by weak VdW; boiling points increase with increasing chain length due to increased surface contact b/n molecules and more VdW forces

• differs by CH2

• insoluble in polar solvents like water as they are non-polar molecules and don’t form hydrogen bonds with water, but they are soluble in non-polar solvents such as cyclohexane or chloroform

• unsaturated: bromine water is red-brown to colourless or dilute solution of KMnO4 is purple to colourless

• typically undergo ionic addition reactions

• naming: start w/ lowest possible no. to position of double bond

• important isomers: C4H10 & C5H12

ionic addition reactions in alkenes

double bond b/n carbon atoms will become single & molecule added on splits in two and added to a carbon atom

reagents: H2, Br2, Cl2, HCl, H2O, Br2

conditions: Ni catalyst & @ 200 degrees C

compounds formed:

1. ethane

2. 1,2 dibromoethane

3. 1,2 dichloroethane (1st stage of 3 stages in the conversion of ethene gas to PVC (plastic)

4. 1 chloroethane

5. ethanol

6. 2,3 dibromobutane

aliphatic hydrocarbon: alkynes

• triple bond b/n two of the C atoms

• -yne

• ethyne is gaseous as VdW forces are very weak; so it’s a small molecule w/ smaller surface area

• differs by CH2

• insoluble in water as they are non-polar molecules and don’t form hydrogen bonds with water, but they are soluble in non-polar solvents such as cyclohexane or chloroform

• unsaturated: decolourise in bromine water & dilute KMnO4

• typically undergo ionic addition reactions

• ethyne used for cutting and welding metals

• combustion: C2H2 + 2 ½ O2 → CO2 + H2O

• in air = sooty luminous flame // in pure oxygen = luminous flame

chloroalkane

an alkane where one or more of the hydrogen atoms have been replaced by chlorine atoms - don’t occur naturally

alcohol - w/ hydroxyl group

• an alkane where on of the hydrogens is replaced by an -OH group

• -anol

• general formula is CnH2n+1 OH

• tetrahedral & -OH group is V-shaped

• -OH polar while the rest of carbon chain is non-polar

• end of C chain, replace H by -O-H

• higher boiling points than alkanes of similar Mr due to hydrogen bonds b/n -OH groups due presence of OH; therefore more energy is needed to break these bonds

• this hydrogen bonding b/n alcohol molecules also account for the fact that the first 4 members are liquids

classification of alcohols

1. primary alcohols - when there is 1 carbon directly bonded to the carbon atom w/ -OH group attached

2. secondary alcohols - when there are 2 carbons directly bonded to the carbon atom w/ -OH group attached

3. tertiary alcohols are not easily oxidised

who has tetrahedral carbons

all alkanes, chloroalkanes and alcohols as each carbon in these molecules form 4 single bonding pairs and no lone pairs

solubility of alcohol in water

• smaller carbon chains = soluble due to h-bonding b/n molecules

• as c-chain increases in length, the non-polar carbon chain takes over from polar -OH group, so solubility decreases

• hence larger alcohols become readily soluble in non-polar solvents

reactions of alcohols

1. act as acids w/ extremely reactive metals such as Na to form a salt and hydrogen gas

2. primary alcohols can be oxidised to aldehydes and then on to form carboxylic acids. secondary alcohols can be oxidised to ketones. oxidising agent used is acidified sodium dichromate (look at redox reactions)

3. alcohols react with carboxylic acids to form esters and water - esterification reaction / condensation reaction (look at saponification experiment)

4. alcohols can undergo elimination reactions (dehydration) to form an alkene and water

ethanol

• fermentation via enzyme (zymase)

• sugars such as glucose converted to c2h5oh + 2co2

• all alcohols are toxic and are broken down in the body by the liver to an aldehyde (primary metabolite)

• uses: solvent for perfumes + sterilisation

homologous series: aldehydes (contains at least one planar carbon in their structure)

1. homologous series containing the -CHO functional group

2. ending = -anal

3. contain a carbonyl group in their structure; C=O. This carbonyl group is  polar due to the difference in electronegativity values of the carbon and oxygen atoms. The oxygen is more electronegative and the carbon is less electronegative

4. As a result of the carbonyl group there will be dipole-dipole attractions between the aldehyde molecules making them have higher boiling points than corresponding alkane molecules BUT lower boiling points than corresponding alcohol molecules (why?) 

5. Methanal is a gas, while ethanal is a volatile liquid at room temperature N.B.

6. It is worth noting that the functional group is always written at the right of the molecule and the carbon of the functional group is always  when naming. The usual rules for naming apply.The lower aldehydes are soluble in both water and non-polar solvents such as cyclohexane. The reason why the lower aldehydes are soluble in water, a polar solvent, is due to hydrogen bonding that occurs between the oxygen of the carbonyl group of the aldehyde and the hydrogen of water. The higher aldehydes are insoluble in water as the non-polar carbon chain of the aldehyde molecule has greater influence.

7. aromatic aldehyde - benzaldehyde is an aromatic aldehyde that is found in almond oil of almond kernels

homologous series: ketones

1.        Ketones have the general formula , where  and  are alkyl groups which may be the same or different. ( and  cannot be hydrogen!!!)

1.        Ketones also contain the carbonyl group as their functional group, -C=O and so the physical properties show a lot of similarities to aldehydes.

2.        The boiling points of ketones are higher than that of corresponding alkanes due to the dipole-dipole attractions between the polar carbonyl groups but again lower than corresponding alcohols.

3.        The lower ketones are soluble in polar water due to hydrogen bonding between the oxygen of the carbonyl group of the ketone and the hydrogen in water. Like the alcohols and aldehydes, the solubility in water decreases as the length of the non-polar carbon chain increases. Ketones are soluble in non-polar solvents like cyclohexane.

4.        Propanone is an excellent non-polar solvent itself. It is found in nail varnish remover (old name is acetone).

5.        ending –anone is used

6.        The carbon of the functional group is never the first or last carbon of the molecule. So its position is indicated by the smallest possible number and then the usual rules apply when naming.

7.        It is important to note that the first member of the homologous series is propanone as ketones can only have a minimum of 3 carbons in the chain. There is no need to use the number in the name for propanone and butanone, as there is only one possible location for the carbon of the functional group regardless of which direction you name the molecule from. However, pentanone has pentan-2-one and pentan-3-one. Draw them to see the difference.

 

 

homologous series: carboxylic acids

1.        Carboxylic acids have a general formula of , where R is either hydrogen or an alkyl group.

2.        The functional group is -.

3.       The boiling points of carboxylic acids is considerably higher than the corresponding alcohols due to the hydrogen bonding present between the molecules caused by the polar –OH group present in the functional group. The molecules also group together in twos (called dimers) held together by hydrogen bonds. As a result, more energy is needed to break these hydrogen bonds, hence the highest boiling points of all the homologous series.

4.      The carboxylic acids are liquids at room temperature also due to the hydrogen bonds holding the molecules together.

5.        The lower carboxylic acids are soluble in polar water due to the formation of hydrogen bonds with water. The oxygen of the carbonyl group of the acid forms a hydrogen bond with the hydrogen in water and the hydrogen of the –OH group of the acid forms another hydrogen bond with the oxygen of water. As the length of the carbon chain increases (non-polar part) increases, solubility in water decreases. The higher members of the acids are soluble in non-polar solvents such as cyclohexane.

6.       When naming carboxylic acids the functional group is always on the right hand side of the molecule hence is  and if the molecule has groups attached they must be named from this end. The endings are –anoic acid i.e. methanoic acid, ethanoic acid, propanoic acid etc.

7.      Methanoic acid (formic acid) is found in nettle and ant stings. Ethanoic acid used in the manufacture of cellulose acetate, which is used in varnishes and is also found in vinegar. Propanoic acid and benzoic acid (aromatic) are used as food preservatives.

8.        Methanoic acid is unusual as it also can act like aldehydes as its structure also contains the aldehyde functional group. We will discuss this later when examining reactions of the carboxylic acids!

 

homologous series: esters

1.        Esters have a general formula of , where  is either hydrogen or an alkyl group and  is an alkyl group. The functional group of the esters is circled in the diagram.

2.    Esters are compounds formed when the hydrogen of the functional group of a carboxylic acid is replaced by an alkyl radical of an alcohol.

3.        They are formed by the reversible reaction between a carboxylic acid and an alcohol. This is known as a CONDENSATION REACTION –

4.      They have relatively low boiling points due to dipole- dipole attractions between the molecules.

5.      The lower esters are soluble in polar water as the oxygen of the carbonyl group forms hydrogen bonds with the hydrogen of water. Solubility decreases as the length of the carbon chain increases. The higher esters are soluble in non-polar solvents such as cyclohexane.

6.     Naming esters is more complicated than usual so we will give this a separate section to itself.

7. ending is -anoate

8. R1 = carbon from carboxylic acid & R2 comes from alcohol

9. they’re named backwards; alkyl group at end of molecule comes with and front is given an -anoate ending

condensation reaction

a condensation reaction occurs when two different molecules combine to form a more complex molecule with the production of a smaller molecule such as .

polyesters

POLYMERS that consist of hundreds of thousands of ester molecules linked together under certain conditions of temperature and pressure.

used in the clothing industry and other fabric industries

fats & oils

fats and oils are esters that occur naturally. An example of one such ester is GLYCERYL TRISTEARATE

perfumes

esters have distinct aromas

solvents

ethyl ethanoate is used as a non-polar solvent

redox reactions

• The preparation of an aldehyde, ketone and a carboxylic acid are redox reactions as the alcohol is oxidised and the oxidising agent (sodium dichromate), is reduced during the preparation.

• The reduction of the dichromate ion during these reactions leads to a colour change of orange to green as Cr(+6) is reduced to Cr (+3)

so… (→ = oxidised)

primary alcohol → aldehyde → carboxylic acid

secondary alcohol → ketone

 

diagnostic tests to show the presence of aldehydes via reactions of ethanal (excellent reducing agents): ethanal w/ acidified potassium permanganate

·       Place a few  of ethanal into a test tube with some acidified potassium manganate (VII).

·       Heat gently in a water bath for a few minutes.

Result – the purple colour of the manganate (VII) goes colourless as the ethanal is a reducing agent and reduces Mn +7 to Mn +2

equation: CH3CHO + Mn+7 → CH3COOH + Mn 2+

manganate (vii) solution should be acidified because we want to provide H+ ions (?)

diagnostic tests to show the presence of aldehydes via reactions of ethanal (excellent reducing agents): Fehling’s solution

·       Place a few  of ethanal into a test tube.

·       Add equal volumes of Fehlings solution A+B (blue in colour due to presence of Cu (II)).

·       Heat gently and shake to mix.

Result – the blue colour of the Fehlings solution changes to a red precipitate. This is due to ethanal acting as a reducing agent which reduces Cu2+, which is blue to  Cu + is a red ppt.

CH3CHO + Cu2+ → CH3COOH + Cu+ (if ethanal is reducing agent, then it gets oxidised to ethanoic acid)

diagnostic tests to show the presence of aldehydes via reactions of ethanal (excellent reducing agents): w/ Tollen’s reagent (ammonical silver nitrate solution) aka the silver mirror test

·       Place a few  of silver nitrate solution into a test tube. Add a small amount of NaOH

·       Add a few drops of ammonia (aqueous) until the silver oxide ppt disappears.

·       Add a few drops of ethanal and warm gently.

Result – a deposit of metallic silver is seen on the inside of the test tube.

This is due to the silver in Tollens Reagent which has an oxidation number of Ag +1 being reduced by ethanal to metallic silver which has an oxidation number of Ag 0

equation: CH3CHO + Ag+ → CH3COOH + Ag (↓)

preparation of a ketone

if oxidising a secondary alcohol, we would produce a corresponding ketone; bc they’re not easily oxidised, so need to use the same precautions such as an excess of alcohol over oxidising agent

reduction of aldehydes and ketones

1.        Aldehydes are reduced to the primary alcohol using reagent - hydrogen and condition - a nickel catalyst.

The equation for the reaction is:

CH3CHO + H2 —(ni)—> C2H5OH

 

2.        Ketones are reduced to the secondary alcohol using reagent - hydrogen and condition - a nickel catalyst.

The equation for the reaction is –

CH3COCH3 + H"2 → CH3CH(OH)CH3

reactions of ethanoic acid: w/ sodium carbonate (diagnostic test for carboxylic acids)

·       A spatula load of sodium carbonate was placed into a test tube.

·       A few  of ethanoic acid was added.

Result - Effervescence was observed as carbon dioxide was given off.

This carbon dioxide quenched a lighted taper held at the mouth of the test tube.

Equation for the reaction is –

2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O (sodium ethanoate)

reactions of ethanoic acid: w/ magnesium metal (not a diagnostic test)

·       A test tube was quarter filled with ethanoic acid.

·       About 3 of magnesium ribbon was added.

Result – the magnesium ribbon disappeared and hydrogen gas was given off.

Equation for the reaction is-

2CH3COOH + Mg → (CH3COO)Mg + H2 (magnesium ethanoate)

reaction of ethanoic acid: w/ ethanol (esterification reaction

·       When a carboxylic acid and an alcohol are reacted together under reflux in the presence of conc. sulfuric acid, an ester is formed.

·       This reaction is referred to as an esterification reaction. It may also be called a substitution reaction as the hydrogen of the –OH group of the acid is replaced by an alkyl group from an alcohol. It can also be called a condensation reaction as two different molecules react to produce a more complex molecule with the production of a smaller molecule such as water. When ethanoic acid and ethanol are refluxed together the ester ethyl ethanoate is formed.

 

 

The equation for the reaction is –

 

                 CH₃COOH     +     C₂H₅OH  ⇌CH₃COOC₂H₅     +      H₂O

 

As we can see this is a reversible reaction and so is affected by changes of concentration and temperature (but not pressure, why?).

The purpose of the concentrated sulfuric acid is two fold –

1.        It acts as a dehydrating agent that causes the concentration of the water to decrease. This drives the position of equilibrium to the right, favouring the forward reaction so more ester, ethyl ethanoate, is produced.

2.        It also acts as a catalyst for the reaction.         

reduction of carboxylic acids

Carboxylic acids are reduced to firstly an aldehyde then to a primary alcohol using hydrogen and a nickel catalyst.

equation:

CH3COOH + H2 → CH3CHO + H2 → C2H5OH