ALKANES AND CYCLOALKANES

2ND SHIFTING - ORGCHEM LEC

Reaction Mechanisms

- subtype of chemical kinetics

- overall progress of chemical reaction

-stepwise chemical reaction (or the sequence of elementary steps that lead to product formation)

Types of Chemical Kinetics

reaction rate and reaction mechanism

General Organic Chemistry Reactions

Addition, Elimination, Substitution, Rearrangement

Addition is also known as

Combination, Synthesis, Direct Union

Addition

A + B -> AB

Elimination is also known as

Decomposition, Analysis

Elimination

AB -> A + B

Substitution is also known as

Displacement

Types of Substitution

Single, Double

Single Displacement

AB + X -> AX + B

Double Displacement is also known as

Metathesis, Exchanges

Double Displacement

AB + CD -> AC + BD

Rearrangement is also known as

Cis/Trans Interconversion

Example of Rearrangement

Keto-Enol Tautomerization

Common General Reaction Mechanisms

Polar and Radical

Polar

• results from a heterolytic bond cleavage/formation (e- pair move together)

• interaction between Nucleophile (Nu) and Electrophile (e+)

Nucleophile (Nu)

• neutral/negatively charged atom

• e- rich species

• e- pair donor

Examples of Nucleophiles

HO-, H2O

Electrophiles are also known as

Lewis Acids

Electrophile (e+)

• neutral/positively charged atom

• e- poor species

• e- pair acceptor

Examples of Electrophile

C=O, R-X

Radical Mechanism

• results from a homolytic cleavage/formation

• e- pair move independently -> radical formation (Reactive Oxygen Species)

• common reaction mechanism for alkanes/cycloalkanes in which a radical is formed at high temperature (△) or irradiation (hv)

Reactive Oxygen Species (ROS)

can damage cells/tissues

Polar Mechanism results to

(A-) Nucleophile/Nu

(B+) Electrophile/e+

Radical Mechanism results to

(A.) Free Radical

(B.) Free Radical

Terms to Remember When Describing a Reaction Energy Diagram

Activation Energy, Transition State, Delta G naught prime, Fast/Slow, Exergonic reaction, Endergonic reaction

Activation Energy (Ea)

minimum energy required to reach the transition state

Transition State (Ts)

state of peak free energy and where reactants start to form

Transition State is also known as

Activated Complex, Intermediate Products (unstable species)

Delta G naught prime (△G°)

• free energy change of a reaction in standard condition

• change in Gibbs free energy

Fast/Slow

• amount of Ea to reach Ts

• inversely proportional with Ea

Ea = molecular collisions = Ts formation = _ reaxtion rate

low, high, high, high

Exergonic (Exothermic) Reaction

• reactions that releases energy (hot surroundings)

• has a negative △G° (△G° < 0)

Endergonic (Endothermic) Reaction

• reactions that absorbs energy (cold surrounding)

• has a positive △G° (△G° > 0)

Fast and Exergonic with a negative △G°

Slow and Exergonic with a negative △G°

Fast and Endergonic with a positive △G°

Slow and Endergonic with a positive △G°

First Transition State

a=

Carbocation Intermediate

b=

Second Transition State

c=

Carbocation Intermediate

In cases of 2-step reaction, a ______ is formed in between 2 transition states (Ts).

Carbocation Intermediate

require energy for activation in order to progress thru the reaction.

CnH2n+2

General Formula of Alkanes

Alkanes

saturated hydrocarbons made up of completely single bonds

Petroleum and Natural Gas

sources of alkanes and a mixture of hydrocarbons

Kinds of Alkanes

• Linear-chain/Normal

• Branched-chain

CnH2n

General Formula of Cycloalkanes

Cycloalkanes

saturated hydrocarbons (HC) but has 2 fewer hydrogens compared to normal alkanes

Cycloalkanes

• have their terminal carbons connected to created an enclosed cyclic system

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

insoluble in water

PHYSICAL PROPERTIES OF ALKANES AND CYCLOALKANES:

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

lower densities than water

PHYSICAL PROPERTIES OF ALKANES AND CYCLOALKANES:

They will not dissolve in water and furthermore they will even float.

BP increases = carbon chain increases in length or size

PHYSICAL PROPERTIES OF ALKANES AND CYCLOALKANES:

As the carbon chain increases, the number of IMF also increases which is required to be broken for a material to boil.

least

Alkanes and Cycloalkanes are the ____ reactive of all organic compounds but are not completely unreactive.

Combustion

CHEMICAL PROPERTIES OF ALKANES AND CYCLOALKANES:

reaction between a substance and O2 that produces CO2 and H2O, resulting in the production of large amounts of heat and light; less dangerous to environment

CO2, H2O, Heat and Light Energy

Result of Combustion

Cracking

CHEMICAL PROPERTIES OF ALKANES AND CYCLOALKANES:

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

Heat/Al2O3

Catalyst of Cracking

smaller/shorter alkanes

Result of Cracking

Types of Cracking

Thermal, Catalytic

Oil refining

use of cracking wherein crude petroleum is distilled to separate it into usable fractions that differ in BP

increases

Stability _____ from primary to quaternary, primarily because the R-groups provides support on stability when a carbocation is formed.

Primary carbon / 1○

Secondary carbon / 2○

Tertiary carbon / 3○

Quaternary carbon / 4○

substitution-type reaction

Alkanes and Cycloalkanes will only react under specific conditions of

temperature and irradiation following a _______.

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

• follows a radical mechanism

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

R-X (halogenated alkanes / haloalkanes)

Primary Product of Halogenation

H-X

By-Product of Halogenation

Initiation Phase/Radical Production

High temperature/light (irradiation) is required to break the bonds X-X bonds. This results into the formation of X• radical.

Radical/Free radical is an atom with an unpaired electron; on the product side

Propagation Phase

The Cl• radical initiate the homolytic cleavage of one C-H bonds by attracting an electron to itself.

It results in the formation of a HCl and a new •CH3 (methyl radical).

Termination Phase/Non-Radical Production

Combination of 2 radicals causes that reaction to undergo R• the termination phase. It decreases the available radicals for propagation.

mono-, di-, or poly- substituted products

Radical substitution (SR) of alkanes can form __________ because halogenated alkane can continuously react with halogens resulting to polyhalogenated products.

excess (x’ss) amount of alkane

To maximize monohalogenation and prevent continuous propagation of

radical species, __________ is used.

Reactivity-Selectivity Principle

In situation, where there are multiple carbons that can undergo

halogenation, the _______ is applied.

Reactivity-Selectivity Principle

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

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

5.0x

Relative reactivity of alkyl radical formation by a Cl• at RT: 3°

3.8x

Relative reactivity of alkyl radical formation by a Cl• at RT: 2°

1.0x

Relative reactivity of alkyl radical formation by a Cl• at RT: 1°

1600x

Relative reactivity of alkyl radical formation by a Br• at 125°C: 3°

82x

Relative reactivity of alkyl radical formation by a Br• at 125°C: 2°

1.0x

Relative reactivity of alkyl radical formation by a Br• at 125°C: 1°