organic mechnisms. meet the attackers press the space bar

ORGANIC MECHNISMSORGANIC MECHNISMS

MEET THE ATTACKERSMEET THE ATTACKERS

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I AM A NUCLEOPHILE

I HAVE A LONE PAIR WHICH I CAN USE TO FORM A NEW BOND.

I ATTACK ELECTRON DEFICIENT AREAS (those with a + or )


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I AM AN ELECTROPHILE

I HAVE A + CHARGE or a CHARGE. I ATTACK ELECTRON

RICH AREAS SUCH AS C=C DOUBLE BONDS.


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AND I AM A FREE RADICAL

I HAVE AN UNPAIRED ELECTRON WHICH I WANT TO PAIR UP. I AM

VERY REACTIVE AND DON’T MIND WHERE I ATTACK… WATCH OUT!

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

WHO IS ATTACKED?WHO IS ATTACKED?


SINGLEA typical covalent bond with one shared pair – nothing to tempt an attacking species



SINGLE

MULTIPLE

A typical covalent bond with one shared pair – nothing to tempt an attacking species

Bond has twice as many electrons – species which like electrons will be attracted



SINGLE

MULTIPLE

NON-POLAR



Similar atoms have an equal attraction for the shared pair of the covalent bond



SINGLE

MULTIPLE

NON-POLAR

POLAR



Similar atoms have an equal attraction for the shared pair of the covalent bond

Atoms have different electronegativities and the shared pair will be attracted more to one end – species known as nucleophiles will be attracted to the slightly positive end


ALKANES ARE RELATIVELY UNREACTIVE ORGANIC COMPOUNDS

ALKANES CONTAIN TWO BOND TYPES

C-H and C-C

WHAT ATTACKS ALKANES?WHAT ATTACKS ALKANES?

Both bonds are single – no electron rich areas

Bonds are non-polar - no electron deficient areas

Free radicals are very reactive and do attack

FREE RADICAL SUBSTITUTIONFREE RADICAL SUBSTITUTION

ALKENES ARE MUCH MORE REACTIVE THAN ALKANES

ALKENES CONTAIN A C=C BONDThere will be twice as many electrons between the carbon atoms as there are in a single bond

WHAT ATTACKS ALKENES?WHAT ATTACKS ALKENES?

C=C bond is double – an electron rich area

Bonds are non-polar - no electron deficient areas

ELECTROPHILIC ADDITIONELECTROPHILIC ADDITION

HALOGENOALKANES ARE MUCH MORE REACTIVE THAN ALKANES

HALOGENOALKANES CONTAIN A POLAR BONDThe halogen is more electronegative than the carbon attracts the shared electron pair.

WHAT ATTACKS HALOGENOALKENES?WHAT ATTACKS HALOGENOALKENES?

Bond is single – not an electron rich area

Bond is polar – the greater electronegativity of the halogen creates a dipole making the carbon atom electron deficient.

NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

CURLY ARROWSCURLY ARROWS



ELECTRONS MOVE FROM AREAS OF HIGH ELECTRON DENSITY TO ONES WITH A LOWER ELECTRON DENSITY




For example…from LONE PAIRS

DOUBLE BONDS




For example…from LONE PAIRS

DOUBLE BONDS

to POSITIVE SPECIESTHE END OF POLAR BONDS


ARROWS WITH TWO ‘HEADS’ INDICATE THE MOVEMENT OF TWO (A PAIR OF) ELECTRONS





A PAIR of electrons moves from here… to here





ARROWS WITH ONE ‘HEAD’ INDICATE THE MOVEMENT OF JUST ONE ELECTRON






ONE electron moves from here… to here





ALWAYS BE PRECISE WITH THE POSITIONING OF ANY ARROWSALWAYS BE PRECISE WITH THE POSITIONING OF ANY ARROWS


ONE electron moves from here… to here

NUCLEOPHILES:- possess a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS



HYDROXIDE ION

negative charge

H Olone pair

more lone pairs



HYDROXIDE ION

negative charge

H Olone pair

more lone pairs

AMMONIA MOLECULE

Nucleophiles don’t need to have negative charge… BUT they must have a lone pair

lone pair

H

H N

H

ELECTROPHILES:- attract a lone pair of electrons




HYDROGEN ION HThere are no electrons in the outer shell of hydrogen so it has space to accept two electrons



HYDROGEN ION HThere are no electrons in the outer shell of hydrogen so it has space to accept two electrons

HYDROGEN CHLORIDE

contains a POLAR BOND; the end will attract the electron pair H Cl


When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

This is the basic mechanism for the nucleophilic substitution of bromoethane.




H O H C C Br

H

3

H

H C C O H

H

3

H

Br

This is the basic mechanism for the nucleophilic substitution of bromoethane. To see how it works, it helps to show the electrons involved.




H O H C C Br

H

3

H

H C C O H

H

3

H

Br




H O

The hydroxide ion has a – ive charge, The oxygen has eight electrons in its outer shell. Because it has a lone pair, the ion will act as a nucleophile




H O H C C Br

H

3

HIn bromoethane, C2H5Br, the C-Br bond is polar because the electronegativity of Br is greater than C. It will susceptible to attack by nucleophiles.

In addition reactions, 2 or more substances react to form a single product only

ADDITION REACTIONSADDITION REACTIONS

C2H4(g) + Br2(l) ——> CH2BrCH2Br(l) ethene 1,2 - dibromoethane

Reactant 1 + Reactant 2 Product

Example : Bromine undergoes addition reaction with ethene

Reactant Product 1 + Product 2

ELIMINATION REACTIONSELIMINATION REACTIONS It is opposite of addition reaction. One substance react to form 2 products

C3CH2OH ————> CH2=CH2 + H2O ethenol ethene

Example :Dehydration of ethenol

CONDENSATION REACTIONCONDENSATION REACTION Involves addition reaction followed by an elimination reaction Two reactants combine to form a larger molecule with the elimination of a small molecule like water

CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)

ethanol ethanoic acid ethyl ethanoate + water

Example : formation of ester from carboxylic acid

SUBSTITUTION REACTIONSUBSTITUTION REACTION

One atom or group of atoms replaced by another atom or group of atoms

Reactant 1 + Reactant 2 Product 1 + Product 2

OXIDATION AND REDUCTIONOXIDATION AND REDUCTION reactions when oxidation or deduction takes place

Example : oxidation of primary alcohol to form aldehyde

HYDROLYSISHYDROLYSIS Splitting up a molecule by reacting with water

CH3CH2Br + H2O CH3CH2OH + HBr

Example :hydrolysing of bromoethane

Note[O] means oxidising agent

POLYMERISATIONPOLYMERISATION

Joining of small (monomers) together in to a long chain 2 types of polymerization are there . They are Adition polymerisation and condensation polymerization

Polymer could be represented by :-[M]n-

There are 3 ways to split the shared electron pair in an unsymmetrical covalent bond.

UNEQUAL SPLITTINGproduces : IONS (negative ions are calledCarbanion. Positive ions are called Carbocation)known as HETEROLYTIC FISSION

EQUAL SPLITTINGproduces RADICALSknown as HOMOLYTIC FISSION

• If several bonds are present the weakest bond is usually broken first • Energy to break bonds can come from a variety of energy sources - heat / light • In the reaction between methane and chlorine either can be used, however... • In the laboratory a source of UV light (or sunlight) is favoured.

HOMOLYTIC AND HETROLYTIC FISSIONHOMOLYTIC AND HETROLYTIC FISSION

The formation of a carbocation from a halogenoalkane is an example of ??

heterolytic fission.

Free radical – species with an unpaired electron Electrophile – species that accepts a pair of electrons which are attracted to region of higher electron density Nucleophile – species that donates a pair of electrons which attacks region of lower electron density

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