CARBON CHEMISTRY

Topic 11: HSC course

should be practical where possible and students should be familiar with the laboratory techniques commonly used in organic chemistry (refluxing, distillation, BP determination, separating aqueous and non-aqueous layers). Students should be able to write structural formulae, give systematic names for the compounds they study, and to write equations for reactions.

1. IUPAC nomenclature

IUPAC nomenclature for compounds containing up to 10 carbon atoms.

(a) alkanes, alkenes, alkynes and their halogen derivatives (including dienes)

(b) cycloalkanes and cycloalkenes

(c) alkanols, alkanals, alkanones and alkanoic acids (including diols and triols)

(d) esters

draw structural formulae (using condensed structural formulae is acceptable) for compounds containing up to 10 carbon atoms

isomers (excluding geometric and optical)

2. Hydrocarbons -- alkanes and alkenes

structure and physical properties

chemical reactions, involving alkanes and alkenes:

(a) oxidation -- combustion, formation of diols and cleaving the double bond

(b) substitution reactions

(c) addition reactions -- only H2, X2, HX (not Markovnikov)

description of the industrial and domestic uses of common hydrocarbons

a) methane, propane and butane

b) ethene

safety precautions in relation to hydrocarbons and haloalkanes.

3. Alkanols

structure and physical properties

fermentation of sugars to form ethanol

oxidation to form alkanals, alkanones and alkanoic acids -- using KMnO4 and K2Cr2O7

esterification of alkanols

industrial and domestic applications of:

(a) ethanol

(b) antifreeze: 1,2 -- ethanediol

(c) glycerol: 1,2,3 -- propanetriol.

4. Mandatory experiences

refluxing of an organic reaction

chemical tests to differentiate between alkanes and alkenes (using Br2 and KMnO4)

oxidation of alkanols (primary, secondary and tertiary -- using KMnO4 and K2Cr2O7).

5. Suggested experiences

fermentation of a sugar to form an alkanol

a distillation to separate organic components in a solution

esterification of short chain organic acids

construct models of various organic compounds.

Notes for Exam

• Dashes and commas in systematic naming 2,3-dimethyl pentane.
• Give place of double bond. 2-pentene.
• Watch x10^-X. Don't go wrong way.
• Check chemical equations in exams: balanced?, got subscripts?
• Watch solids in equations for equilibrium (s) and liquids (l).
• Remember pH of OH- is > 7. Eg Ba(OH)2
• Ethanoic acid needs water to become acidic because pure (glacial) acetic acid does not contain any water to react with the CH3COOH and form hydronium ions. Acidic hydrogen in glacial acetic acid is locked in the molecule and is not ionised;

Some molecule have a functional group at both ends of a molecule, eg ethanedioic acid, butanedioic acid.

Key Terms

Functional Group: An atom or group of atoms which gives a homologous series its characteristic chemical properties.

Homologous Series: A series where all the members have the same functional group and the same general formula. Members differ by a -CH2 group.

2. Hydrocarbons -- alkanes and alkenes

structure and physical properties

Hydrocarbon molecules are held together by weak dispersion forces. Dispersion forces become stronger as the size of molecules increase.

As branching increases this interferes with the packing together of molecules and lowers the melting point, boiling point and density.

chemical reactions, involving alkanes and alkenes:

(a) oxidation -- combustion, formation of diols and cleaving the double bond

(b) substitution reactions

(c) addition reactions -- only H2, X2, HX (not Markovnikov)

PROPERTIES OF ALKANES

Formula CnH2n+2

• Combust to give CO2 and H2O as products, C or CO if limited oxygen
• Alkanes are non-polar, only attracting is weak dispersion forces
• Either gasses (1-4 carbons), low BP liquids (5-20 carbons) or soft solids (>20 carbons). Reason increasing size of molecules
• Naming of branches is alphabetical before numerical, eg 4-ethyl-2-methyl.

Reaction of Alkanes

• Alkanes (saturated hydrocarbons) react with Halogens in substitution reactions when exposed to UV light.
• eg: CH4 + Br2 -> CH3Br + HBr.
• Further substitution can occur (until CBr4, tetrabromo-methane).

Alkane + Halogen ->(uv) Haloalkane

Uses of Alkanes

• Methane - natural gas; home heating.
• Propane - LPG, bottled gas; cooking heating.
• Butane - lighter fuel
• Octane - petrol
• C12 -> C15 - kerosene.

PROPERTIES OF ALKENES

Formula CnH2n

• Unsaturated molecules, degree of saturating measured by Hydrogen:Carbon ratio.
• Alkenes have slightly lower MP and BP. Why? Weaker dispersion forces, less molecules

Chemical reactions of Alkenes

• Alkenes are more reactive than alkanes. Why? The double bond, site of high electron density.
• Alkenes have addition reactions
• Addition reactions are faster and require less energy than substitution reactions.
• Hydrogenation, H2 - addition of hydrogen
• Halogenation, X2 - addition of halogen (F, Cl, Br, I).
• Hydrochlorination, HCl - Produces 2 isomers
• Hydration, H2O - Addition of water (Needs Acid Catalyst).



• When testing for double bonds with Br2 or MnO4, UV light is not needed as the reactants are much more active than the alkanes.

Oxidation Reactions of Alkenes

Oxidation reactions involve oxygen being added to the molecule or Hydrogen being removed.

Oxidation of Alkenes

Oxidising agents

Oxidising agents such as acidified potassium permanganate (H+/KMnO4) and acidified potassium dichromate (H+/K2Cr2O7) are used because they change colour when they are reduced. ie

MnO4^- H⁺ + e^- --> Mn₂+ + H₂O

purple brown

Cr₂O₇ + H⁺ + e^- --> Cr₃+ + H₂O

orange green

(note: equations are not balanced)

Uses of Alkenes

• Ethene, C2H4 - manufacture of polyethylene plastic bags. Polymers

ALKYNES

The high density of bonds in the triple bond means that alkynes will undergo addition reactions like alkenes. While the ethene decolourise bromine water with ethyne it takes a few minutes to disappear.

Reaction involving Alkynes

• Hydrogenation, H2 - forms ethene, than ethane, (needs 150 C and Ni catalyst)
• Hydrohalogenation, HX - eg forms 1,1 dicholoroethane
• Halogenation X2 - Ethyne and chlorine gas explode when mixed. -> 2HCl + 2C(s).
• Halogenation with Br2 water - forms 1,2 dibromoethene, then 1,1,2,2 tetrabromoethane.
• Oxidation, H+/KMnO4 (cold/dilute). Ethyne -> 1,2 ethendiol (-OH added to each side of carbon triple bond) -> 1,1,2,2 ethantetraol (-OH added to each side of carbon triple bond)
• Oxidation, H+/KnMnO4 (hot/concentrated) ethyne -> 2 molecules of methanoic acid

Additions reactions are used to test for unsaturated hydrocarbons:

Decolourising yellow Br2(aq)

Decolourising purple MnO4-(aq)

Reaction to make ethyne:

CaC₂ + 2H₂O -> H-CC - H + Ca(OH)₂

Calcium Carbide

Oxidation - combustion, formation of diols, and cleaving the double bond

Combustion

Alkanes and alkenes combust in air releasing energy. The products are H2O and depending on the amount of oxygen CO2, CO or C (soot).

Formation of Diols

Alkenes oxidise in cold permanganate solution to form diols.

H H H H H

C - C = C - C - C - H --> [O], H2O

H H H

(2 - pentene)

H H H H H

--> H - C - C - C - C - C - H

H OH OH H H

2,3 - pentandiol

Alkanes do not react with aqueous MnO4-

Cleaving the double bond

Hot acidified permanganate solution cleaves the double bond in Alkenes forming alkanone groups and/or acid groups.

e.g. 2 - butene oxidises to form 2 molecules of ethanoic acid.

2 - pentene oxidises to form a molecule of ethanoic acid and a molecule of propanoic acid.

If a 1-alkene is oxidises in this way the end carbon becomes a carbon dioxide.

E.g. 1 - pentene forms butanoic acid and carbon dioxide.

CYCLIC HYDROCARBONS

Carbons form a closed ring.

Cyclic Alkanes

Have the formula CnH2n

• Are saturated

Cycloalkenes

CnH2n-2

HALOALKANES

Compound in which on or more hydrogen atoms have been replaced by halogen atoms are called haloalkanes.

Use bromo-, chloro-, etc...

3. Alkanols

ALKANOLS

structure and physical properties

Have -OH functional group

CnH_2n+1OH

Position isomerism is possible because -OH group can be located on different carbons. ie

Alkanols are polar molecules and can hydrogen bond to one another. Thus they have much higher boiling points.

The first three alkanols - methanol, ethanol propanol - are infinitely soluble in water. As the size increases, they become less soluble as the polar OH group makes up less of the molecule.

How do alkanols dissolve both polar and non-polar compounds?

Alkanols dissolve in non-polar solvents because of the non-polar part of the molecule uses dispersion forces to attract the other non-polar molecules.

Alkanols dissolve in water because the polar part of the alkanol bonds to the polar water molecules. Hydrogen bonding.

fermentation of sugars to form ethanol

Complex starches can be broken down into glucose. Glucose is broken down by yeast to form ethanol and CO2.

C₆H₁₂O₆ ->[^{yeast enzymes}] 2C₂H₅OH + 2CO₂

Oxidation of Alkanols

oxidation to form alkanals, alkanones and alkanoic acids -- using KMnO4 and K2Cr2O7

The products of oxidation using aqueous solutions of oxidising agents [O] such as acidified permanganate H+/MnO4- or acidified dichromate H+/Cr2O7- depend on whether the alcohol is primary, secondary or tertiary.

1. Primary

Primary alcohols RCH2OH have the hydroxy on a carbon attached to one other carbon.

Primary alcohols are oxidised to an alkanal and then an alkanoic acid.

Chemical Reaction:

Primary alkanol -> [O] alkanal ->

[O] alkanoic acid

R-CH₂OH -> [O] R-CHO + H2O -> [O] R-COOH

2. Secondary

Secondary alcohols RRCHOH have the hydroxy on a carbon attached to two other carbons.

Secondary alcohols are oxidised to an alkanone.

Chemical Reaction:

Secondary alkanol -> [O] alkone

RCHOHR₁ -> [O] RC=OR₁ + H2O

3. Tertiary

-OH on carbon which is attached to three other carbons.

Chemical Reaction:

Tertiary alkanol -> no reaction

Colour changes when oxidising agents reduce:

MnO4- --> Mn2+

(purple) (brown)

Cr2O72- --> Cr3+

(orange) (green/blue)

Alkanols have a higher MP and BP than corresponding alkane. Why?

Alkanes are only held together by dispersion forces only which are relatively weak.

Alkanols are held together by hydrogen bonding between the O and the H on the two different Alkanols.

The presence of the -OH group results in hydrogen bonding between ethanol molecules, giving higher MP and BP.

Alkanols with small chains (< 6 carbons) tend to be soluble in water and non-polar solvents. This property makes alkanols versatile solvents.

The solubility in water decreases as the carbon chain becomes longer.

Alkanols with active metals

Active metals react with alkanols to form a salt and hydrogen gas.

Alkanol + Sodium -> Sodium Alkoxide + Hydrogen

C₂H₅OH + Na -> C₂H₅O-Na+ + �H₂

ethanol sodium ethoxide: salt

These salts are very basic and will hydrolyse in water to form an alkanol and a strong base.

C₂H₅O^-Na⁺ + H₂O -> C₂H₅OH + NaOH

Ethanol

Note: -lyse = split

Substitution reactions of Alkanols

Alkanols can react in substitution reactions with hydrogen halides (HX) or phosphorous halides (PX₃).

Reactions with hydrogen halides (HX) are reversible while those with phosphorous halides (PX₃) are not.

Dehydration of Alkanols

Alkanols can undergo dehydration (having water molecule removed by heating with concentrated H2SO4.

DIOLS AND TRIOLS

Diols are compounds which contain 2 -OH groups (hydroxyl groups).

Triols are compounds which contain three hydroxyl groups.

ESTERS

esterification of alkanols

Esters are compounds formed by reacting an alkanoic acid with an alkanol using a concentrated H2SO4 catalyst. This is called esterification. (Hydrophilic - water loving)

The purpose of the H2SO4 catalyst is to remove the H2O molecule.

Esterification is a particular type of condensation reaction in which two molecules combine with the loss of a water molecule.

Refluxing is used to increase the yield of an ester by:

• heating reactants above their boiling points
• recovering gaseous products and reactants through condensation.

Step 1: Refer to drawing of condenser (remember water in at bottom) and pear shaped flask. Step 2: separating flask.

Prac: Esterification

Condenser. Mixing Alkanol and Alkanoic acid.

industrial and domestic applications of:

(a) ethanol

(b) antifreeze: 1,2 -- ethanediol

(c) glycerol: 1,2,3 -- propanetriol.

4. Mandatory experiences

Refluxing of an organic reaction = esterification

2. Alkenes decolourise

3.

Fermentation

The fermentation mixture can be distilled to separate the ethanol from the yeast/water mixture using the different boiling points of ethanol and water.

Explain how antifreeze (1,2 ethandiol) would lower the boiling point of water. Include structural diagrams.

The 1,2 ethandiol disrupts the bonding between water molecules which does not allow the water to form a crystal structure. The normal structure cannot form because of the H-bonding between water and the -OH groups, which disrupts the normal H-bonding between water molecules.

Visualise diagram.

Alkanoic Acids