Topic 20 - Organic chemistry Introduction- functional groups and naming Nucleophilic substitution reactions Elimination reactions Condensation reactions Reaction pathways Stereoisomerism

20.1 Ester Functional group: -COOC- Condensation reaction or esterification Many fruit-smelling esters The bond in triglycerides (lipids) Alcohol + carboxylic acid  ester + water CH3-OH + HOOC-CH3  CH3OOC-CH3 + H2O (The underlined forms water)

Esterification Name: => Methyl-buthanoate Butanoic acid Methanol Name: The alcohol part: Stem + yl: Methyl- The acid part: the salt name; -buthanoate => Methyl-buthanoate

Which names?

Which names? Ethylethanoate propylethanoate 2-butylethanoate penthylpropanoate

Amine Relatives to ammonia => weak bases Functional group –NH2 H-bonds => higher bp’s, smaller ones are water-soluble etc. Name: stem + suffix: -ylamine (or prefix amino-) Methylamine CH3-NH2 Ethylamine CH3-CH2-NH2

Amide Functional group: -CONH Name: stem + suffix: -anamide Methanamid H-CONH2 Ethanamid CH3-CONH2 Peptide bond in proteins

Nitrile Functional group: -CN Former name: cyanides Name: stem + suffix: nitrile Metanenitrile: HCN Ethanenitrile: CH3-CN

Nucleophiles and electrophiles- often needed in organic reactions Nucleophile- nucleus lover Has free electronpair and whole or part negative charge The larger the negative charge - the better the nucleophile Eg: C=C, H2O, -OH, -CN, NH3 Electrophile-electron lover Has whole or part positive charge The larger the positive charge - the better the Electrophile Eg: C=O, H+, C-Cl,

20.2 Nucleophilic substitution reactions Nu: + R-X  Nu-R + X: It’s important to know the difference of these because they will undergo different forms of nucleophilic substitution reactions

SN2 or SN1? SN2 SN1 Bimolecular = two species in the rate determining step. Rate = k [org]*[Nu] Primary halogenoalkanes Steric hindrance One-step with transition state Inversion of configuration Monomolecular = one species in the rate determining step. Rate = k [org] Tertiary halogenalkanes Heterolytic fission of substrate rate determining step Formation of inermediate carbocation Racemix mixture formed

Factors affecting the rate The halogen in the halogenoalkane is important for the reaction rate: C-I > C-Br > C-Cl > C-F (cf. bond strength) The stronger the nucleophile, the faster the SN2-reaction CN- > OH- > NH3 > H2O

SN2 substitution reaction with ammonia H3N: + R-X  R-NH2 + HX Forming an amine

SN2 substitution reaction with CN- as nucleophile R-X + CN-  R-CN + X- Forming a nitrile The carbon chain will be prolonged with one carbon atom

Reactions with nitriles Nitrile + H2  Ni catalyst  Amine CH3CH2-CN + H2/Ni  CH3CH2-NH2    Nitrile + Acidic water  Carboxylic acid CH3CH2-CN + H+/H2O  CH3CH2-COOH

20.3 Elimination reactions CH3CH2-Br + OH-  CH2=CH2 + H2O + Br- Condition: Hot and concentrated sodium hydroxide solution in ethanol. Reflux. Opposite to addition reaction 2 kinds; E1 and E2 If the conditions are different: A substitution reaction CH3CH2-Br + OH-  CH3CH2-OH + Br- Will happen in warm dilute solution of sodium hydroxide, the hydroxide ion will work as a nucleophile.

The E1 mechanism (similar to SN1) Involves heterolytic fission of C-X bond Tertiary halogenalkane Via a carbocation intermediate

The E2 mechanism (similar to SN2) Concerted bimolecular reaction Primary halogenalkane Via transition state

Dehydration reaction of alcohol to form alkene E1 or E2? Excess of H2SO4, ~170oC

20.4 Condensation reaction 2 Organic molecules  1 Organic molecule + water Acid catalysed Esterification, etherification Common in biochemistry, formation of polysaccharides, proteins, nucleic acids

Esterification Name: R-yl R’-oate H2O Name: R-yl R’-oate Esters: flavouring agents (food, perfume), solvents, plastics (polyesters)

Amide condensation Amine Carboxylic acid Amide Water H2O

Polycondensations Plastics Polyesters: polyethylene terphtalate (PET) Polyamides; nylone

Monomers with two functional groups are required Benzene-1,4-dicarboxylic acid + Ethane-1,2-diol Can condense with a diol Can condense with a dicarboxylic acid Monomers with two functional groups are required

Polyamides HOOC-R-COOH + H2N-R’-NH2  H2N-R’-NH-OC-R-COOH Di-amine amide bond HOOC-(CH2)4-COOH + H2N-(CH2)6-NH2   HOOC-(CH2)4-CONH-(CH2)6-NH2 + water hexanedioic acid + 1,6-diaminohexane  Nylon

Reaction pathways 1. Elimination reaction. Hot, concentrated and reflux 2. Substitution reaction. SN1 or SN2 3. Substitution reaction. SN1 or SN2. (Can be substituted up 4 times to a quarternary ammonum salt) 4. Condensation reaction. Acid catalyst (or alkaline catalyst, but more common when hydrolysis of ester). Equilibrium reaction. 5. Condensation reaction. Acid catalyst (or alkaline catalyst). Equilibrium reaction. 6. Nitrile to amin: Reduction with H2 and Ni-catalyst

Stereoisomerism Different location in space of atoms or groups

Structural isomers Chain: e.g. n-butane : methylpropane Positional: e.g. 1-propanol : 2-propanol Functional groups: Ethanoic acid, CH3-COOH : Methyl methanoate HCOOCH3 Propanal, CH3-CH2-CHO : Propanone, CH3-CO-CH3

Geometric Double bond = p and s bonds. No free rotation around a double cis-2-buten trans-2-buten cis-1,2-dichloroethane: bp = 60.3oC trans-1,2-dichloroethane: b.p. = 47.5oC

cis-but-2-ene-1,4.dioic trans-but-2-ene-1,4.dioic m.p = 286oC m.p.= 130oC with decomposition. Strong H-bond between molecules in trans. Strong H-bond in the molecule in cis.

Cyclic compounds can also give geometric isomers Cis and trans

Optical mirror plane enantiomers to each other

Stereo isomers Different location in space. Geometric cis-2-buten trans-2-buten Optical mirror plane enantiomers to each other

A carbon with 4 different substituents is said to have an asymmetric centre or to be chiral. Chiral can also mean the whole molecule. Enantiomers have the same physical properties except that they rotate the plane of polarised light in different directions. Very important which form of the enantiomer in biology and medicine.

Polarimeter Light: Electromagnetic radiation. Normal light oscillating in all directions. Plane-polarized light: When normal light is sends throuhg a polarizing filter only waves in the same plan can pass. If two polarizing filters places 90o to each other the light will be compleataly blocked.   Polarimeter: Measure how much the light will rotate when pass through a solution with optically avtive substance. A sample holder between two filter