Structure and Reactivity | Organic Chemistry 1

Structure and reactivity in organic chemistry are studied in this chapter: the three classes of reagents (Bronsted acids/bases, Lewis acids/bases, nucleophile/electrophile), the chemical functional groups, the naming of alkanes (an example of naming in organic chemistry), introduction to isomerism.

Bronsted Acids/Bases

A Bronsted acid is a proton donor.
A Bronsted base is a proton acceptor.

 

 

The more stable a base, the weaker the base and the stronger the conjugate acid.

Effects that affect relative acid and base strengths:

- Resonance Forms: the more forms a chemical compound has, the more stable it is

- Hybridization Effect: lone pairs of electrons are stabilized by the s character of orbital. The % of s character is 25% in sp3 hybridization, 33% in sp2 and 50% in sp. Therefore the increasing basicity correlates to hybridization the following way sp < sp2 < sp3

- Inductive Effect: bases are stabilized by electron-withdrawing inductive effect (-I effect)

 

Lewis Acids/Bases

A Lewis acid is a substance that accepts an electron pair.
A Lewis base is a substance that donates an electron pair.

 

Nucleophile vs. Electrophile

Nucleophile: an electron rich species that can form a bond by donating a pair of electrons to an electron poor atom. Nucleophiles may be either neutral or negatively charged. They are Lewis bases.

Electrophile: an electron poor species that can form a bond by accepting a pair of electrons from a nucleophile. Electrophiles may be either neutral or positively charged. They are Lewis acids.

Functional Groups

A functional group is a group of atoms that has a characteristic chemical behavior.

 

Carbonyl Group (Cpd with a C=O bond):

Naming Alkanes

Alkanes: Only C-C and C-H single bonds (saturated hydrocarbons).
General formula: CnH2n+2

 

Straight-chain alkanes:

CH4 = methane
CH3CH3 = C2H= ethane
CH3CH2CH3 = C3H8 = propane
C4H10 = butane
C5H12 = pentane
C6H14 = hexane
C7H16 = heptane
C8H18 = octane
C9H20 = nonane
C10H22 = decane

If a H is removed from an alkane, the partial structure is called alkyl and is part of larger compounds. To name them, we replace the suffix 'ane' by 'yl'.

 

CH3- = methyl
CH3CH2- = ethyl
CH3CH2CH2- = propyl

 

Some other common substituents:

 

Naming: Prefix-Parent-ane 
Prefix = Substituents, Parent = Longest chain of C atoms, -ane = alkane chemical function

1) Find the longest continuous chain of C atoms. If a molecule has two chains of equal length, choose the chain with the largest number of substituents. 

2) Name all groups attached to the longest chain as alkyl substituents.

3) Number the C atoms of the longest chain beginning with the end that is closest to a substituent. If there are 2 subst. at equal distance: lower number for the first subst. in alphabetical order. If the 2 subst. are the same, check the 3rd one.

4) Write the name as a single word, using hyphens to separate the different prefixes, and commas to separate numbers. Cite the subst. in alphabetical order (di, tri, tetra ... not counted).

 

1) Longest chain: 7 C atoms = heptane
2) 2 methyl subst. + 1 ethyl subst.
3) The right end is the closest to a subst.: number the C atoms from the right to the left
4) 4-ethyl-3,3-dimethylheptane

 

Isomers vs. Conformers

Constitutional isomers: same molecular formula but different connectivity.

 

 

 Conformers = Conformational Isomers: specific arrangements of atoms that result from rotation about a single bond.

 

Conformers of 1,2-dichloroethane.
Newman Projection (view the C-C bond directly end-on).