Chemistry: Intro to Org Chem, Isomerism and Reaction Mechanisms

learning outcomes: (a) interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: (i) hydrocarbons (alkanes, alkenes and arenes) (ii) halogen derivatives (halogenoalkanes and halogenoarenes) (iii) hydroxyl compounds (alcohols and phenols) (iv) carbonyl compounds (aldehydes and ketones) (v) carboxylic acids and derivatives (acyl chlorides and esters) (vi) nitrogen compounds (amines, amides, amino acids and nitriles) (b) describe sp3 hybridisation, as in ethane molecule, sp2 hybridisation, as in ethene and benzene molecules, and sp hybridisation, as in ethyne molecule (c) explain the shapes of, and bond angles in, the ethane, ethene, benzene, and ethyne molecules in relation to  and  carbon-carbon bonds (d) predict the shapes of, and bond angles in, molecules analogous to those specified in (c)

sp³ hybridisation

one s and three p orbitals combine to form 4 equivalent hybrid orbitals

shape: tetrahedral about C atom

bonds: 4 sigma bonds

bond angle: 109.5 degrees

sp2 hybridisation

one s and two p orbitals combine to form 3 equivalent hybrid orbitals

shape: trigonal planar about C atom

bonds: 3 sigma bonds, 1 pi bond

bond angle: 120 degrees

sp hybridisation

one s and 1 p orbitals combine to form 2 equivalent hybrid orbitals

shape: linear about C atom

bonds: 2 sigma bonds, 2 pi bonds

bond angle: 180 degrees

why is that C=C bond length shorter than C-C

C=C, both carbon atoms are sp2 hybridised, bond is formed by sp2-sp2 overlap.

C-C, both carbon atoms are sp3 hybridised, bond is formed by sp3-sp3 overlap.

there is higher percentage of s character in sp2 (33%) than sp3 (25%).

thus e- of sp2 are closer to the nucleas thus bond is shorter (and stronger).

how are sigma and pi bonds formed in (insert hybridised molecule)

sigma: head on overlap of sp3/sp2/sp hybrid orbitals (with 1s orbital of H, if the molecule got H)

pi bonds: side on overlap w adjacent unhybridised p orbitals

aldehyde

CHO

ketone

C=O

there is no H.

acyl chlorides

COCl

amine

RNH2 or R2NH or R3N

amides

CON(H)

amino acid

amine and carboxylic acid are attached to the same carbon

true or false: constitional isomers may differ in their chemical properties

true. constitutional isomers can have diff functional grps, thus chemical properties

cis vs trans isomers

cis: same grp attached to same side

trans: same grp attached to diff side

in pic attached, the same grp refers to the H atom

how to know if a compoud exhibits cis-trans isomerism?

1. Cis–trans isomerism arises from restricted rotation around a C=C bond due to the π bond

2. occurs when each carbon in the bond is attached to two different groups.

no. of cis-isomers formula

2^n where n = no of double bonds with different sets of groups on each end

why can’t the above structure exhibit cis-trans isomerism despite having C=C

C=C in the ring: cant form trans isomer due to ring strain of a small ring structure

C=C in side chain: one of the C atoms is attached to two identical H atoms

racemic mixture

1:1 ratio of two enantiomers in the mixture

why are smell detecting receptors in our nose chiral?

only one enantiomer with the correct spatial arrangment can bind to one particular receptor resulting in different smells being detected

constituitional isomerism

same molecular formula but different structural formula

why does mpt of cis isomer differ from trans isomer?

mpt of trans > cis

trans isomers are more closely packed than cis [thus, larger surface area of contact —> stronger idid, more energy to overcome. this is for simple covalent molecules]

chiral molecule

has a chiral centre (aka the centre is attached to four diff grps) + no internal plane of symmetry

what entails a pair enantiomers?

a chiral molecule and its non-superimposable mirror image. (like our hands)

total number of stereoisomers

2^n where n = no. of chiral centres + no. of C=C bond or C–C bond (part of a ring structure) with cis-trans isomerism.

why does cis and trans isomer bpt differ?

cis: polar, pdpd, trans: non-polar, idid

pdpd stronger than idid, thus more energy to overcome

true or false: enantiomers have the same physical properties

partially true. they have identical physical properties except they rotate plane–polarised light in opposite directions.

rue or false: enantiomers have the same chemical properties

they have identical chemical properties except towards chiral reagents

why does a racemic mixture not rotate plane-polarised light

1. pair of equal amounts of enantiomers will rotate the plane of polarised light by the same extent but in opposite directions

2. cw rotation caused by one enantiomer is exactly cancelled by the acw rotation of the other enantiomer as the.

3. no net effect on the passage of plane-polarised light and the sample is optically inactive.

what forms a carbocation

heterolytic fission

nucleophiles

electron rich species, can donate a lone pair of electrons to form a new bond

(attack the atom with partial positive charge)

addition reaction

two or more moleculse combine to form a single product, involves breaking of pi bonds

elimination reaction

Removal of atoms or groups of atoms from two adjacent atoms to form a pi bond

condensation reaction

two functional groups from different molecules react with each other to form a larger molecule and a small molecule (usually H2O) is released as a byproduct

extra info: other than water, sometimes ammonia and hydrogen chloride may also be a byproduct

hydrolysis

adding water to one larger molecule to break it into smaller molecules.

(usually acids or bases are catalysts)

how does a carbocation become stable when theres an alkyl group

alkyl grp exerts electron donating inductive effect which helps to disperse the positive charge on the carbocation, stabilising the carbocation.

degree of substitution

primary carbon - attached to one other carbon atom

secondary - 2C, tertiary - 3C, quaternary - 4C

carbocation

ion with a positive charge on the carbon atom

electrophile

electron poor, can accept an electron pair to form a new bond

free radical

species with an unpaired electron, thus highly reactive

homolytic fission

covalent bond breaks, two shared electrons split equally to form free radicals

half arrow

heterolytic fission

two share electrons split unequally. one atom keeps both electron and the other is electron deficient. full arrow. show the charge on the products.

resonance effect

three of more adjacent p orbitals overlap, so pi electrons move over a greater region in spacs, aka pi electrons are delocalised

steric hindrance

occurs when large groups on a molecule get in the way and thus hinder the reaction

electron donating inductive effect

let G - e- withdrawing grp. then electrons move towards G. can stabilise anion (disperse negative charge), destabilise cation

electron donating inductive effect

let G - e- donating grp. then electrons move away from G. can stabilise cation (disperse +ve charge), destabilise anion