Structure and Reactivity | Organic Chemistry 1

Structure and reactivity in organic chemistry are studied in this chapter: Bronsted acids/bases, acid strength, Lewis acids/bases, nucleophiles and electrophiles, chemical functional groups, types of isomers, types of arrows, heterolytic and homolytic cleavage

Bronsted Acids / Bases

Bronsted acid:

A Bronsted acid is a proton donor and therefore contains a hydrogen atom. A loss of proton from the acid forms its conjugate base. Common examples of Bronsted acids are HCl, H2SO4, H3O+, acetic acid (CH3COOH), p-toluenesulfonic acid (TsOH)
 

Bronsted base:

A Bronsted base is a proton acceptor. It must be able to form a bond to a proton by donating an available electron pair. A gain of proton by a base forms its conjugate acid. Common examples of Bronsted bases are HO-, RO-, H2N-, R2N​​​​​​​-, H-

 

Acid Strength

A strong acid readily donates a proton, forming a weak conjugate base. The more readily an acid donates a proton, the smaller its pKa, the stronger it is considered. An acid can be deprotonated by the conjugate base of any acid having a higher pKa
 

Base stability:

The more stable a base, the weaker the base and the stronger the conjugate acid. The effects which affect the stability of a base are:

  • Resonance effects: 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 the orbital (25 % in sp3, 33 % in sp2, and 50% in sp). Therefore, the increase in basicity correlates with hybridization as follows: sp < sp2 < sp3
  • Inductive effect: bases are stabilized by electron-withdrawing inductive effect (-I effect)
     

How to compare the acidity of 2 acids:

  1. Draw the conjugate bases

  2. Determine which conjugate base is more stable

  3. Determine which acid is the strongest: the more stable the conjugate base, the more acidic the acid

 

Compare the acidity of the 2 following acids:
 

 

1. Draw the conjugate bases:


 

2. Compare their stability:


A stabilized by resonance effect

3. Determine the strongest acid:

 

Lewis Acids / Bases

Lewis acid:

A Lewis acid is an electron pair acceptor. It must have an unfilled valence shell, a partial positive charge or a proton. Common examples of Lewis acids are BF3, AlCl3, +CR3, H+
 

Lewis base:

A Lewis base is an electron pair donor and therefore contains a lone pair or a π bond. It donates this electron pair to any electron deficient compound. Some common Lewis bases are -OH, -OR, NH3, NR3-NH2, H2O

 

Nucleophile vs. Electrophile

Nucleophile:

An electron-rich species which can form a covalent bond by donating a pair of electrons to an electron-poor atom. A nucleophile can be neutral or negatively charged and is usually symbolized by Nu-
Lewis bases are nucleophiles. Lone pairs and π bonds are nucleophilic sites
 

 

Electrophile:

An electron-poor species which can form a covalent bond by accepting a pair of electrons from a nucleophile. An electrophile can be neutral or positively charged and is usually symbolized by E+
Lewis acids are electrophiles. Electronegative heteroatoms like N, O, or halogens X create electrophilic carbon atoms
 

Functional Groups

Functional group: 

An atom or group of atoms with characteristic chemical and physical properties. It is the reactive part of the molecule


 

Carbonyl group (compound with a C=O bond):

Types of Isomers

Constitutional isomers:

Chemical species with the same molecular formula but which differ in the way atoms are connected to each other. They are also called structural isomers

 

Stereoisomers:

Isomers with the same molecular formula, the same connections but which differ in the way atoms are oriented in space (different 3D geometry). Hashed-wedged line structures are used to depict the 3D arrangement

cis-isomer: isomer with 2 substituents on the same side of the ring or double bond
trans-isomer: isomer with 2 substituents on opposite sides of the ring or double bond

Types of Arrows

Reaction arrow: drawn between the starting materials and products in an equation

Double reaction arrows: drawn between the starting materials and products in an equilibrium equation

Double-headed arrow: drawn between resonance structures

Full-headed curved arrow: shows movement of an electron pair

Half-headed curved arrow: shows movement of a single electron

Heterolytic vs. Homolytic Cleavage

Heterolytic cleavage:

Bond breakage in which the bond electron pair is unevenly divided between products. A normal double-barbed arrow shows the movement of the pair of electrons. Heterolytic bond cleavage of a neutral molecule results in the formation of a cation and an anion

 

Homolytic cleavage:

Bond breakage in which the bond electron pair is evenly divided between products. A single-barbed arrow shows the movement of a single electron. Homolytic bond cleavage results in the formation of radicals