Basic concepts- core organic chemistry

What is isomerism?

Compounds which have same molecular formula but be different

what goes first

carbonyl group is the highest priority

• Remember: There is no need to number an aldehyde as it is automatically carbon-1

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What are the different type of isomers

• Structural isomerism

• Stereoisomerism

What is structural isomerism?

Compounds with same molecular formula but w. diff structural formula

What are the different types of structural isomerism?

• Chain isomerism

• Positional isomerism

• Functional group isomers

Chain e.g.

Pentane and 2-methlybutane

diff branching

What is positional isomerism?

Contain same functional group but in different positions on carbon chain

What is functional group isomerism?

Compounds which have same molecular formula but different functional groups

e.g, aldehydes and ketones

molecular formulae

shows actual numbers of each atom in the molecule

empirical formulae

shows numbers of each atom in simplest whole number ration

general formulae

simplest formulae for a member of a homologous series

displayed formulae

shows every atom and every bond

structural formulae

Shows how atoms are joined together

CH3-CH2-CH2-CH3

hydrocarbon definition

compound of only hydrogen and carbon

saturated definition

containing single bonds only

unsaturated definition

containing multiple carbon-carbon bonds

homologous series definition

a series of organic compounds with same functional group therefore similar chemical properties but with each successive member differing by CH2

functional groups

part of organic molecule largely responsible for its chemical reactions

aliphatic

carbon atoms are joined to each other in unbranched (straight) or branched chains or non-aromatic rings (alkenes, alkanes, alkynes)(not ring

alkynes

containing at least one triple carbon-to-carbon bond

alicyclic

containing carbon atoms joined together in a ring that is not aromatic, with or without branches

aromatic

some or all the carbon atoms are found in a benzene ring

stem =

n. of carbons in longest continuous chain

what n. of carbons = stem

1- meth

2-eth

3-prop

4-but

5-pent

6-hex

7-hept

8-oct

9-non

10-dec

prefix=

added before stem to indicate side chain or functional group

naming aliphatic alkanes (straight carbon + branched)

1. Identify longest continuous chain - corresponding alkane

2. Identify side chains (alkyl group)- count n. of carbon atoms from end

3. Add n. before alkyl groups to show position on parent alkane + dash + name of branch

4. If multiple chains put names in alphabetical order. if on same branch use, di, tri

e.g.

3-methlyhexane

4-ethyl-2,2-dimethylheptance

If two or more possible chains of same length how do you choose a chain

chain with most branched is considered the longest

Separate words and numbers with ____

Separate numbers with a ____

dash

comma

How to find name of alicyclic alkanes?

1- Identify longest continuous chain of carbon atoms

2- Add prefix cyclo-

e.g. cyclopentane

naming alkenes

Use smallest number between double bonds

eg. but-1-ene

but-2-ene

4-methyl,pent-2-ene

alcohol

functional group: -OH

prefix: hydroxy-

haloalkane

Functional group: -Cl, -Br, -I

Prefix: chloro-, bromo-, iodo-

aldehyde

Functional group: -CHO

suffix: -al

e.g. propanol

ketone

Functional group: -C(C0)C-

suffix: one

Carboxylic acid

Functional group: -COOH

Suffix: -oic acid

ester

functional group: -COOC-

Suffix: -oate

amine

functional group: -NH2

prefix: amino-

suffix: -amine

which goes first

alphabetical order

ol should get smallest number

When naming branched chain alcohols, be careful to number the longest possible carbon chain first.

what should have smallest number

ketone

e.g. of structural formula

CH3CH2CH3

skeletal for benzene + molecular

C6H6

How to draw double bond in skeletal form

2 parallel lines

formation of a sigma bonds

• Covalent bond is sharing pair of electrons

• Orbitals overlap, they form covalent bond

• end-on overlap of orbitals results in sigma bond

• each carbon in an alkane forms four sigma bonds

What is shape of alkanes and why

tetrahedral arrangement around each carbon atom and bond angle of 109.5°

beacuse eahc carbon is surrounded by four electron pairs in four sigma-bonds

What does sigma bond in alkanes act as?

axes around whcih atom can rotate freely, so shapes are not rigid

As chain length of alkanes increases of alkanes what happens to b.p?

increases

Why does chain length lead to increased b.p

• n. of electrons increases which increases chain length

• leads to more induced dipole-dipole interactions

• greater london forces, more energy required to separate molecules

In straight chain molecules what happens to surface area and why?

greater surface area in contact possible between molecules

as chain increases in length

therefore more energy needed to overcome it

What does greater surface area in contact mean for b.p?

increases and greater in

branching if carbon chain _____ b.p

deacreases

why does increased branching increase b.p.

less SA of contact between molecules therefore weaker london forces

branches can get in way and prevent branched molecules getting as close together as straight chained molecules, decreasing IMF further

Reasons for lack of reactivity of alkanes:

• C-C and C-H sigma-bonds are strong

• C-C bonds are non-polar

• Electronegativity of carbon and hydrogen is so similar the C-H bond can be considered non-polar

Complete combustion- exces oxygen:

alkane + oxygen —> carbon dioxide + water

equation for balancing complete combustion of alkanes

combustion of alkanes is highly ____ useful beacuse

exothermic

release a relativly large amount of energy per gram of fuel

Incomplete combustion- oxygen is limited:

alkane + oxygen —→

carbon + water

or

carbon monoxide + water

alkanes with chain lengths of ___ carbon atoms are used as fuels in internal combustion engines

5-10

2 types of bond fission (breaking cov bonds in reactions)

-Homolytic

-Heterolytic

What is homolytic fission?

each of atoms in covalent bond takes one of the shared pair of electrons

H3C-CH3 —> H3C· + ·CH3

What does homolytic fission form?

2 radicals

What are radicals

an atom/group of atoms with an unpaired electron. they are v reactive

What is heterolytic fission?

One of atoms in cov bond takes both electrons from shared pair

H3C-Cl —> H3C+ + Cl-

What does heterolytic fission form?

• 2 ions are formed

• atoms that took the pair of electrons forms a neg ion

• atom withut pair of electrons forms a pos ion

In heterolytic fission is arrow double headed or single header

double- shows pair of electrons moving

In homolytic fission is a single headed arrow or double headed arrow used

single- show movement of one electron

What is a substitution reaction?

Atom or group of atoms is replaced by another atom or group

How do alkanes react with halogens?

UV radiation

CH4 +Br2 —> CH2Br + HBr

What happens in substitution reactions?

replacement of hydrogen atoms with halogen to form hydrogen chloride and mixture of haloalkanes

What are the three steps of radical substitution?

1- Inititation- forming a radical

2- Propagation - new radical is made

3- Termination- two radicals comb

1: Initiation

Reaction is started when cov bond in bromine molecule is broken by homolytic fission.

UV splits chlorine molecule into two chlorine radicals

Cl-Cl —> Cl· + Cl·

2: Propagation

Radicals are v. reactive. When Cl radicals collide with methane molecule with sufficient energy, a hydrogen atom is removed to form methyl radical and a molecule of hydrogen chloride.

Cl· + CH4 —> HCl + ·CH3 (methyl radical)

.Reaction propagates through 2 steps, a chain reaction:

• Bromine radical, Br·, reacts with C-H bond in the methane, forms a methyl radical, ·CH3, and a molecule of hydrogen bromine, HBr

• Each methyl radical reacts with another bromine molecule, forming organic product, bromomethane, CH3Br, together with bromine radical.

New bromine radical then reacts with another CH4 molecule as in first propagation step, and the two steps can continue to cycle through in a chain reaction

Another propagation reaction

Methyl radicals are also v reactive and will remove a chlorine atom from a chlorine molecule forming another radical

·CH3 + Cl-Cl —> H3C-Cl (chloro methane) + Cl·

3: Termination

2 radicals collide, forming molecule with all electrons paired. There are a number of possible termination steps with different radicals in a reaction mixture

When 2 radicals collide and react, both radicals are removed from reaction mixture, stopping reaction

When do propagation steps stop

When two radicals collide and the overall reaction stops when there are no radicals left

Cl· +Cl· —> Cl2

·CH3 + ·Cl —> CH3Cl

·CH3 + ·CH3 —> C2H6

Products of reaction

• Mixture of products is made during radical substitution reaction due to reactive radicals in propagation step

• As conc of CH3Cl builds an alternative propagation step becomes increasingly likely as a chlorine radical can collide with CH3Cl molecule:

CH3Cl + ·Cl —> ·CH2Cl + HCl

• The ·CH2Cl collides with chlorine molecule:

·CH2Cl + Cl2 —> CH2Cl2 + ·Cl

• Leads to di-, tri-, tetra- substituted methane

CH4 + Cl2 —> CH3Cl, CH2Cl2, CHCl3, C2H6 + HCl

• Produces mixture of mono and poly-substituted chloromethanes, hydrogen chlorine and some ethane

• Presence of ethane proves the reaction proceeds by this mechanism

• In strong sunlight, reaction between chlorine and methane is explosive. Reaction with Br is lower

·CH3 + ·CH3 —> C3H6

• Free radical substitution is of limited use in synthesising haloalkanes since a mixture since a mixture of products is made. If carbon chain is longer this can also lead to halogenation in diff positions

why does Radical substitution produce a mixture of organic products?

further substitution OR produces different termination products OR More than one termination step

substitution at different positions along chain