Alkanes & Cycloalkanes

Organic Chemistry Reactions

Follows either of the two common general reaction mechanisms; Polar Mechanism & Radical Mechanism

Polar Mechanism

• It results from a heterolytic bond cleavage/formation.

• This is a more common reaction mechanism because of the interaction between; Nucleophile & Electrophile

Nucleophile (Nü)

neutral/negatively charged atom (e.g. HO-, H2O).

Electrophile (e+)

neutral/positively charged atom (e.g. C=O, R-X

Radical Mechanism

• It results from a homolytic bond cleavage/formation.

• This is a common reaction mechanism for alkanes/cycloalkanes in which a radical (an atom with a single unpaired electron) is formed at high temperature (𝚫) or irradiation (hv)

Radical

an atom with a single unpaired electron

Activation Energy (Ea)

Minimum energy required to reach the transition state

Transition state (Ts)

A state of peak free energy and where reactants starts to form products.

Delta G naught prime (𝚫G°)

a free energy change of a reaction in standard condition

Fast/Slow

refers to the amount of Ea to reach Ts; inversely proportional with Ea.

Exergonic reaction

reactions that releases energy; has a negative 𝚫G°

Endergonic reaction

reactions that absorbs energy; has a positive 𝚫G°

with a negative 𝚫G°

Fast and Exergonic

with a negative 𝚫G°

Slow and Exergonic

with a positive 𝚫G°

Fast and Endergonic

with a positive 𝚫G°

Slow and Endergonic

Fast, Exergonic, negative ∆G￮

Slow, Exergonic, negative ∆G￮

Fast, Endergonic, positive ∆G￮

Slow, Endergonic, positive ∆G

carbocation intermediate

require energy for activation in order to progress thru the reaction

Alkanes

• General formula: CnH2n+2

• These organic compounds are referred to as saturated hydrocarbons (HC). It is made up of completely single bonds.

Petroleum and natural gas

sources of alkanes and a mixture of hydrocarbons

Kinds of alkanes

linear-chain / normal (n) and branched-chain alkanes

Cycloalkanes

• General formula: CnH2n

• Similar to alkanes, they are also saturated hydrocarbons (HC) but has 2 fewer hydrogens compared to normal alkanes.

• Have their terminal carbons connected to created an enclosed cyclic system.

• They can be substituted with different atoms or functional groups similar to a normal alkane

Linear-Chain Alkane

n-pentane

Branched-chain alkane

2-methylbutane

monosubstituted cycloalkane

methylcyclohexane

disubstituted cycloalkane

1-isopropyl-3-methylcyclohexane

Petroleum and natural gas

sources of alkanes and a mixture of hydrocarbons.

Alkanes & Cycloalkanes

This is insoluble in water

Alkanes & Cycloalkane

Both of them exhibits London dispersion forces which is a non-polar IMF and is not soluble in polar molecule such as water.

Lower density

Alkane & Cycloalkane this is the density in water.

Dissolve in water

This what happens to alkanes & cycloalkanes in water & would float

Boiling points of linear alkanes and cycloalkanes increases

This is as the carbon chain increases in length or size.

the number of IMF (intramolecular forces) increases

• As the carbon chain increases this what happens

• It is required to be broken for a material to boil.

Alkanes & Cycloalkanes

Least reactive of all organic compounds but are not completely unreactive

Combustion & Cracking

This reaction undergoes in Alkane & Cycloalkanes

Combustion

Reaction between a susbtance and O2 that produces CO2 and H2O resulting in the production of large amount of heat and light.

Cracking

Refers to a chemical process where large hydrocarbon molecules (like long-chain alkanes or cycloalkanes) are broken down into smaller, more useful molecules like alkenes and shorter alkanes with the aid of heat and/or Al2O3 catalyst

Stability increases from primary to quaternary

The stability——— primarily because the R-groups provides support on stability when a carbocation is formed

Primary Carbon

Secondary Carbon

Tertiary Carbon

Quaternary carbon

Substitution-type reaction.

Alkanes and Cycloalkanes will only react under specific conditions of temperature and irradiation following this…

Substitution Reactions

Chemical reaction in which part of a small reacting molecule replaces an atom or a group of atoms on a hydrocarbon or hydrocarbon derivative.

Halogenation

Chemical reaction between a substance and a halogen in which one or more halogen atoms are incorporated into molecules of the substance

Radical Mechanism

Radical Mechanisms

R-H (alkane/cycloalkane) demonstrates the -H atom that will be replaced by one incoming -X atom.

R-X (halogenated alkanes / haloalkanes

Primary product/formation of radical mechanism

H-X as a by-product

The -H and -X that was cleaved from their respective reactants will form this

Radical/Free radical

Atom with an unpaired electron

Initiation Phase

Propagation Phase

Termination Phase

mono- , di-, or poly- substituted products

This is formed by radical substitution as halogenated alkane can continuously react with halogens resulting to polyhalogenated products.

Excess (x’ss) amount of alkane

This is used to maximize monohalogenation and prevent continuous propagation of radical species

Reactivity Selectivity Principle

• States that the greater the reactivity of the species, the less selective it will be

• This is applied where there are multiple carbons that can undergo halogenation

• It aids in determining the most common halogenation product that can be formed.