Organic Chemistry

General formula for Alkanes

C_nH_2n+2

1 Carbon

Meth

2 Carbons

Eth

3 Carbons

Prop

4 Carbons

But

5 Carbons

Pent

6 Carbons

Hex

7 Carbons

Hept

8 Carbons

Oct

9 Carbons

Non

10 Carbons

Dec

With 4 valence electrons, what are the 4 possibilities for each carbon atom?

➢ 4 single C–C bonds

➢ 2 single C–C bonds and 1 double C=C bond

➢ 1 single C–C bond and 1 triple C≡C bond

➢ 2 double C=C bonds

What is the definition of fossil fuels?

Fuels (including crude oil and natural gas) formed by natural resources such as buried dead organisms, due to heat and pressure acting on these organisms and anaerobic decomposition of these organisms.

What is the definition of crude oil?

A liquid mixture of different hydrocarbons of different molecular weights (that is found in geologic formations beneath Earth’s surface)

What is the definition of Natural Gas?

A naturally occurring hydrocarbon gas mixture, primarily consisting methane (impurities include carbon dioxide). It is typically found above the crude oil layer.

What is the definition of fractional distillation?

A separation technique that is used to separate a liquid mixture comprising of components with different boiling points.

Answer Format: Predict whether CH4 or C8H18 would have a higher boiling point.

C₈H₁₈ would have a higher boiling point than CH₄.

1. Boiling points increase with increasing M_r due to the larger electron clouds present.

2. This leads to stronger and more extensive intermolecular forces of attraction,

3. and more energy is needed to overcome these intermolecular forces of attraction for larger alkanes.

How does fractional distillation separate crude oil into the various components?

1. At the bottom of the fractionating column, crude oil is heated into vapour.

2. The vapour is pumped into a fractionating column where it is cool at the top but hot at the bottom.

3. As the hot vapour rises, it cools down. The smaller hydrocarbons (i.e. the hydrocarbons with lower boiling points) are collected at the top of the fractionating column (as gases) while the larger hydrocarbons (i.e. the hydrocarbons with higher boiling points) are collected at the bottom of the column.

#Smaller hydrocarbon molecules with lower boiling point due to weaker intermolecular forces of attraction between molecules collect at the top of the fractionating column.

Boiling Points in Fractional Distillation (Top → Bottom)

1. Refinery Gas

2. Petrol

3. Naphtha

4. Kerosene

5. Diesel

6. Lubricating Oil

7. Bitumen

What is the definition of cracking?

The breaking down of long-chain (or bigger) hydrocarbons into smaller molecules.

Usually done under high temperature.

What is the definition of catalytic cracking?

When a catalyst (Al₂O₃, SiO₂) is used to speed up the cracking process

Why is cracking important?

1. to produce petrol (long-chain hydrocarbons are less flammable).

2. to produce short-chain alkenes (which can be used to manufacture other compounds, e.g.

plastics).

3. to produce hydrogen (source of clean fuel) e.g. Toyota fuel cell cars

Eg. of Cracking

C₁₂H₂₆ → C₄H₈ + C₂H₆ + 3C₂H₄

Due to Principle of conservation of mass, Total C and H atoms in reactants must be equal to Total C and H atoms in products.

What is the definition of fossil fuels?

Non-renewable energy sources, which are energy sources that are finite.

Name some biofuels.

Bioethanol

Biodiesel

Biogas

Advantages of biofuel

• Carbon neutral, renewable and sustainable if crops and trees are replanted

• Reduces greenhouse emissions and pollution

• Biofuel production could provide money for less developed countries as they have the space to grow the crops required

• Provides jobs in the agriculture and energy sectors

Disadvantages of biofuel

• Many developed countries don't have the space to be able to produce enough plants to make biofuels because the land is needed for food production

• For bioethanol to be burnt in a car engine, some engine modification is needed, although (most) modern petrol engines can use petrol containing up to ten per cent ethanol without needing any modifications

• Although biofuels are in theory themselves carbon neutral, this does not take into account the carbon dioxide emissions associated with growing, harvesting and transporting the crops, or producing the ethanol from them

What is the definition of saturated and unsaturated compounds?

Saturated compounds have only single (C – C) bonds between their atoms.

Unsaturated compounds have double or triple bonds (π bonds) between their atoms.

What is the definition of a functional group?

an atom or group of atoms common to a series of organic compounds that determines the chemical properties of the series

What is the definition of a homologous series?

a series compounds with the same functional group and same general formula.

What are some similarities of those in the same homologous series?

1. Can be represented by a general molecular formula

2. Differ in formula by a constant –CH₂– unit

3. Exhibit similar chemical properties but different physical properties

General formula for Alkenes

C_nH_2n

General formula for Halogenoalkanes

C_nH_2n+1X, 

where X is –Cl, –Br or –I

General formula for alcohols

C_nH_2n+1OH

General formula for carboxylic acids

C_nH_2n+1COOH

General formula for esters

RCOOR’

Functional groups of alkanes

None

Functional groups of Alkenes

Functional groups of Halogenoalkanes

Functional groups of alcohols

Functional groups of carboxylic acids

Functional groups for esters

What is the definition of a structural formula?

the sequence in which the atoms are bonded to each other, for an unambiguous structure

e.g.

What is the definition of displayed formula/full structural formula?

 the relative placing of all atoms and number of bonds between the atoms in the molecule. All bonds would have to be shown clearly.

What is the definition of a skeletal formula?

derived from the structural formula. When writing the skeletal formula, the H and C atoms are removed, leaving just the C – C bonds in the skeleton and the associated functional groups

What are the types of organic reactions?

1. Addition (2 to 1)

2. Substitution (becomes conjoined with Br, etc)

3. Elimination (1 to 2)

4. Condensation (forms water or other small molecules)

What is the definition of addition?

Reaction where two reagents react to form a single product.

What is the definition of substitution?

Reaction where one atom or a group of atoms is substituted by another atom, group of atoms, or ion.

What is the definition of elimination?

Reaction involving the removal (elimination) of an atom or group of atoms from a single reactant molecule.

What is the definition of condensation?

Reaction involving two or more reactants to yield a product, with the formation of water or some other small molecule

What is the definition of isomer?

compounds that have the same molecular formula but different structural formula.

What are the type of constitutional/structural isomerism?

• same molecular formula

• different structural formula

There are:

1. chain

2. positional

3. functional group

What is chain isomerism?

Different arrangement of carbon chain

e.g. Butane and 2-methylpropane

What is positional isomerism?

Different positions of the same functional group on a carbon chain

e.g. Propan-1-ol and Propan-2-ol

What is functional group isomerism?

Different functional groups present

e.g. ethanoic acid and methyl methanoate

#Carboxylic acids and esters are functional group isomers.

Common Lab Techniques for Organic Reactions

1. Heating under reflux → to minimize loss of reactants due to vapourisation

2. Distillation → used when product is more volatile than reactants + components have different boiling points

3. Extraction using Separatory funnel → used when two solvents are immiscible

4. Recrystallisation → used when compounds in a particular solvent have different solubilities

How are branched chain alkanes formed?

Branched chain alkanes are formed when one (or many) of the H atoms of the straight chain alkanes is/ are replaced with one (or many) alkyl groups.

How are alkyl groups different from alkanes?

Alkyl groups, with general formula -C_nH_2n+1, are named by replacing -ane with -yl.

Alkyl general formula

-C_nH_2n+1

ANSWER:

Using structure, why do alkanes have low melting and boiling points?

Due to their simple molecular structures, low amount of energy is required to overcome the weak van der Waals’ forces of attraction between molecules.

ANSWER: How does boiling point change across alkanes?

• Boiling points of alkanes increase with increasing M_r due to the larger electron cloud size.

• There is greater distortion of electron cloud and stronger, more extensive intermolecular forces of attraction between molecules,

• which requires more energy to be overcome.

ANSWER: Do straight chain alkanes or branched chain alkanes have a higher boiling point?

• Branched chain alkanes have a lower boiling point.

• This is because they have decreased surface area of contact,

• which leads to lower extensiveness of weak intermolecular forces of attraction,

• that requires less energy to overcome.

What is viscosity?

A measure of a fluid’s resistance to a change in shape or to flow.

ANSWER: How does viscosity change through alkanes?

• Viscosity increases as the size of electron cloud increases with M_r

• because the intermolecular forces of attraction between stronger and more extensive

• which causes the liquid to flow less readily

ANSWER: What reactions can alkanes undergo?

1. Combustion

   a. exothermic → used as fuel

   b. complete = CO₂ and H₂O as products

   c. incomplete = CO and H₂O as products

2. Substitution (Halogenation)

   a.

   

   b. can use Cl, Br, I and F

ANSWER: Substitution of Alkanes (halogenation)

• When substitution with (limited) Cl under UV light → decolourisation of greenish-yellow Cl₂ + steamy acidic HCl gas fumes formed

• When further substitution with (excess) Cl under UV light → multiple substituted products

• When substitution with (limited) Br under UV light → decolourisation of reddish-brown Br₂ + steamy acidic HCl gas fumes formed

What is the definition of a cycloalkane?

Cycloalkanes are saturated alkanes with a ring structure and the general formula C_nH_2n.

(shows functional group isomerism with alkenes)

Cycloalkane general formula

C_nH_2n

What is a catalytic converter?

• exhaust system of motor vehicles converts harmful (CO, NO₂, unburnt hydrocarbons) into harmless (H₂O, CO₂, N₂) in presence of suitable catalyst

• Lead poisons/deactivates catalyst so cars with it must be run on unleaded petrol

  NO_x is reduced to N₂ by excess CO with Rh (rhodium) as catalyst

  

  Unburnt hydrocarbons and CO are oxidised to CO₂ and H₂O with Pt and Pd as catalysts

  

What is a parent chain of an alkene?

• longest continuous chain that contains all functional groups

• numbered starting from end closest to double bond

• alkenes with 4 or more carbon atoms exhibit constitutional isomerism

ANSWER:

Using structure, why do alkenes have low melting and boiling points?

Due to their simple molecular structures, low amount of energy is required to overcome the weak van der Waals’ forces of attraction between molecules.

ANSWER: How does boiling point change across alkenes?

• Boiling points of alkenes increase with increasing M_r due to the larger electron cloud size.

• There is greater distortion of electron cloud and stronger, more extensive intermolecular forces of attraction between molecules,

• which requires more energy to be overcome.

ANSWER: Do straight chain alkenes or branched chain alkanes have a higher boiling point?

• Branched chain alkenes have a lower boiling point.

• This is because they have decreased surface area of contact,

• which leads to lower extensiveness of weak intermolecular forces of attraction,

• that requires less energy to overcome.

ANSWER: What reactions can alkenes undergo?

1. Combustion

   a. Burn with sootier flame compared to alkanes because they have a higher percentage by mass of C

   b. complete = CO₂ and H₂O as products

   c. incomplete = CO and H₂O as products

2. Addition

   a. To produce alkanes (hydrogenation)

   b. of halogens

   c. of steam (hydration)

   d. of hydrogen halide (hydrohalogenation)

3. Oxidation

ANSWER: Addition for Alkenes to produce Alkanes (hydrogenation)

ANSWER: Oxidation Test for Alkenes