Organic chemistry – the chemistry of carbon containing compounds

Homologous series

• Family of compounds that differs only by the length of its hydrocarbon chain

• Members share a general formula and chemical properties 

• Examples: alkanes, alkenes, alcohols

• Melting/boiling points increase going down the series OR as LDF increases

  • Boiling point increases due to the increase in number of electrons available to participate in instantaneous dipoles

Formulas & Types of Formulas

There are different ways to draw the same molecule. 

Example: C₄H₈O

//need to know//

• Empirical formula:

• Molecular formula:

• Full structural formula:

• Condensed structural formula:

• Skeletal formula: //don’t need to know how to draw, just how to read it//

Naming straight-chain alkanes (alkane = single bonds)

• Suffix – tells us the functional group of the molecule

  • For alkanes, it’s ‘-ane’

• Prefix – tells us the length of the longest carbon chain

  • 1 carbon → meth-

  • 2 carbon → eth-

  • 3 carbon → prop-

  • 4 carbon → but-

  • 5 carbon → pent-

Examples:

• Ethane → C₂H₃

• Butane → C₄H₁₀

Naming branched-chain alkanes

• Start by naming the longest chain

• Add extras to say the size of a branch, its position, how many of that branch

  • Branch size – same prefixes as before; add ‘-yl’

    • 1 carbon → methyl-

  • Position – based on which carbon in the longest chain it is situated

    • Choose number to minimise the total number used (ie. can be read from left to right and from right to left)

  • Number of same branches:

    • 1 branch → nothing

    • 2 branches → di-

    • 3 branches → tri-

    • 4 branches → tetra-

• Examples:

  • 2-methylpropane

  • 2,3-dimethylbutane

Naming straight-chain alkenes

• Suffix – ‘-ene’

• Prefix – the same as with alkanes

• Add the number where the double bond is situated in the middle of the name

  • Example: pent-1-ene (double bond is found at the first carbon)

Isomers

• Compounds with the same molecular formula, but different structural formula

• Structural isomers – same number of each atom, but bonded in a different order

  • Cyclic alkanes = have a circular shape 

  • Noncyclic alkanes = are linear; have the general formula C_nH_2n+2

Classifying Halogenoalkanes & Alcohols

• Primary, 1°– halogen or hydroxyl group is attached to a single carbon 

• Secondary, 2°– halogen or hydroxyl group is attached to two carbons

• Tertiary, 3°– halogen or hydroxyl group is attached to three carbons

Classifying Amines

• Primary, 1°– nitrogen is attached to one alkyl group

• Secondary, 2°– nitrogen is attached to two alkyl groups

• Tertiary, 3°– nitrogen is attached to three alkyl groups 

Saturated vs Unsaturated Compounds

• Saturated – alkanes (only have single bonds)

• Unsaturated – double/triple bonds

• Aliphatic compounds – straight-chain compounds

• Cycloalkanes – ring structures that contain only single bonds (C-C)

• Arenes – ring structures consisting of alternating double bonds (C=C)

  • Benzene = arene

    • Six carbons with alternating single and double bonds in a ring shape

• Aromatic hydrocarbons contain benzene rings

• Bond length between carbons are of equal length

• Bond order = 1.5

Reactions of Alkanes

Combustion:

• Alkanes are very stable which is why they’re used as fuels

  • They release lots of energy and produce stable compounds

• Complete combustion: 

Alkane + O₂ → CO₂ + H₂O

• Incomplete combustion:

Alkane + O₂ → C (s) + CO (g) + CO₂ + H₂O 

//do not need to include C and CO₂ unless they do not specify that they only want CO//

• Amount of C, CO, and CO₂ will vary depending on conditions

• Alkanes are stable because of their unreactive, nonpolar, single bonds that can break

• ΔH = negative

Substitution Reactions

Free-radical substitution

• Free-radical - species that is formed when a molecule undergoes homolytic fission (2 e^- of a covalent bond are split evenly between 2 atoms resulting in 2 free-radicals that each have a single e^-) 

• Example: 

Halogenation

• Alkanes will undergo halogenation if reacted with a halide in the presence of UV light

  • Example: free-radical substitution

C₂H₆ + Cl₂ →^UV CH₃CH₂Cl + HCl

Radicals

• Species with unpaired electrons ⇒ very reactive

• Halogens form radicals when hit by UV light of the right frequency (Cl₂ →^UV 2Clᐧ)

  • The dot after the Cl represents the unpaired electron and tells us we have a radical 

Reaction Mechanism → Free-radical Substitution

Initiation – radicals formed by homolytic fission

Cl₂ →^UV 2Clᐧ

Propagation – these steps feed each other the radicals needed to continue the reaction

Clᐧ + C₂H₆ → C₂H₅ᐧ + HCl

C₂H₅ᐧ + Cl₂ → C₂H₅Cl + Clᐧ

Termination – any 2 radicals can combine to terminate the reaction; concentration of radicals is low so this is a very rare event 

Clᐧ + Clᐧ →Cl₂; Clᐧ + C₂H₅ᐧ → C₂H₅Cl; C₂H₅ᐧ + C₂H₅ᐧ → C₄H₁₀

• A single radical can cause thousands of cycles of the propagation stage before it reaches termination

• The same mechanism applies to all halogens

• The alkane can be substituted many times, until ever hydrogen is replaced

Reactivity of Alkenes

• Alkenes are considerably more reactive than alkanes and are a major industrial feedstock

  • Their reactivity is due to the double bond which contains 4 e^- (a significant amount of charge)

  • Makes it attractive to electrophiles and enables it to polarise approaching molecules

• Most reactions of alkenes are addition reactions (2 molecules come together to make a new one)

Alkenes & Hydrogen

Alkene + hydrogen → alkane

• Reaction conditions: heat + nickel catalyst

Alkenes & Hydrogen halides

Alkene + hydrogen halide → halogenoalkane

• Reaction conditions: nothing specific

• If the halogen used is an aqueous solution of Bromine, the orange-brown colour of Br solution is decolourised (standard test for alkenes)

Alkenes & Water

Alkene + water → alcohol

• Reaction conditions: water in the form of steam + phosphoric or sulfuric acid catalyst

Polymerisation

• Under the right conditions, alkene molecules will add to each other, creating a polymer

• Required conditions: vary from alkene to alkene + often includes high pressure, temperature and a catalyst

• The carbons in the C=C double bonds form the carbon chain, everything else hangs off this chain

★Naming polymers: name them like alkenes even if they look like alkanes★

Alcohols as Fuels

• Alcohols combust more readily than equivalent alkanes, but release less energy since they are already partially oxidised

Alcohol + oxygen →CO₂ + H₂O

• Alcohols are used as fuels: 

  • For cars, either pure or blended with petrol

  • Methanol as fuel for competitive motorsports (Example: monster trucks)

• Ethical considerations: most fuel ethanol is fermented from crops that could otherwise be eaten, forcing up food prices

Oxidation of Alcohol

• Most important reaction of the alcohols is oxidation

• A range of compounds will oxidise them, so the oxidiser is often represented as [O]

• One oxidising agent is K₂Cr₂O₇ (potassium dichromate), (catalysts are acidic)

  • //need to know// When using this, orange Cr (VI) is reduced to green Cr (III) 

• Tertiary alcohols do not oxidise

Examples:

• Partial – C₃H₈O + [O]→ C₃H₆O + H₂O

• Complete – C₃H₈O + [O] → C₃H₆O₂ + H₂O

There are two steps that occur during the oxidation of alcohol: distillation and reflux. 

• Distillation of a primary alcohol – will result in an aldehyde as the aldehyde has a lower boiling point than the associated carboxylic acid

• Distillation of a secondary alcohol – will result in a ketone 

• Reflux – the process where the aldehyde is not removed from the solution allowing the further oxidation of the aldehyde into a carboxylic acid 

Nucleophilic Substitution

• One of the most important reactions undergone by halogenoalkanes

• A nucleophile is attached to positive charges

  • They either have full negative charges or delta negative charges

  • H₂O and OH^- are both nucleophiles

• The carbon in the carbon-halogen bond has a delta positive charge due to the greater electronegativity of the halogen 

  • Makes it susceptible to attack by nucleophiles

• Carboxylic acid + alcohol → ester