Organic Functional Groups Lecture 3 PDF

ORGANIC FUNCTIONAL GROUPS Dr Graeme Kay Academic Strategic Lead – Chemical Sciences 01224 262548 [email protected] Lecture 3 REACTIVITY Nearly all organic reactions fall into 1 of only 3 main types: Substitutions, Additions or Eliminations Substitutions Many HC reactions C...

ORGANIC FUNCTIONAL GROUPS Dr Graeme Kay Academic Strategic Lead – Chemical Sciences 01224 262548 [email protected] Lecture 3 REACTIVITY Nearly all organic reactions fall into 1 of only 3 main types: Substitutions, Additions or Eliminations Substitutions Many HC reactions Cl + Naare OHof the type: H3C OH + Na Cl 3 Here the OH group is substituting the Cl atom and gives an alcohol product. Note the use of another arrow type!! Means “gives”. These reactions are common in saturated molecules (containing just σ bonds). ADDITION REACTIONS These reactions are characteristic of compounds with multiple bonds (i.e. contain 1 or more π bonds). An example is the reaction of ethene with bromine: Br Br H H + Br Br H C C H C C H H H H Notice here we form one molecule from two individual molecules ELIMINATION REACTIONS These are essentially the opposite of addition reactions. Here, a molecule loses the elements of another small molecule and often we obtain multiple bonded Hproducts. Br H H H C C H C C + HBr H H H H Before we can understand these reactions we first need to understand bond breaking processes. There are two types of bond breaking process. here are two types of bond breaking process. These processes are heterolysis (or heterolytic cleavage) and homolysis (or homolytic cleavage). HETEROLYSIS This is when both electrons of the electron pair of the covalent bond go to one of the fragments. This gives rise to ions. We can see how this happens with a molecule A-B A B A + B This type of bond breaking process takes place when the bond is polar So we have a structure: When we have bonds involving carbon we can obtain a carbocation or carbanion. To obtain a carbocation we have: C Z C + Z carbocation So if we have a carbocation it can react with an anion or molecule with an unbonded pair of electrons: C B C B Because the carbocation is seeking an electron pair it is called an electrophile The ion or molecule with the electron pair that seeks out the + charge of the carbocation is called a nucleophile So we have two very important definitions We can use other terms for electrophiles and nucleophiles that classify them as acids or bases, according to the Lewis definition: acids are electron pair acceptors bases are electron pair donors To obtain a carbanion: Carbocations and carbanions are very reactive because of the structures they possess. Carbocations only have 6 electrons in their valence shell they seek an electron pair from another molecule or ion to achieve the stable octet of electrons HOMOLYSIS With homolytic cleavage of a bond we generate free radicals. A B A + B movement of 1 electron Free radicals are very important in many biochemical pathways and are highly reactive. They are believed to be responsible for a number of disease states, e.g. cancer Acids and bases in organic functional groups We have already seen the use of the words acid and base in organic functional groups: Lewis acid: electron pair acceptor Lewis base: electron pair donor In the Bronsted - Lowry definition we have a different approach: Acid: a substance that donates a proton (H+) to another molecule. Base: a substance that accepts a proton from another molecule. Acids As we have seen these donate a proton to another molecule. So in water we have: H A + H2O H3O + A In organic acids the equilibrium lies far to the left: i.e. they do not readily ionise to lose a H+ ion to water. An equilibrium constant Ka, can be calculated that gives the relative strengths of acids: - [ A ][H O ]  K a 3 [HA ] When the equilibrium lies to the left Ka will be small. It is more convenient to describe acid strength as a pKa value: So the stronger the acid, the lower the value of pKa, Factors affecting acid strength If we consider 2 compounds and their pKa values The reason ethanoic acid is much stronger than ethanol is the nature of the anion formed after the H+ is lost: O O CH3 C CH3 C O O i.e. the anion is resonance stabilised and is therefore easily formed. This cannot happen with ethanol and is a much weaker acid. So a major factor in acid strength is how stable (i.e. how readily formed) the anion is. We will take this further when we look at carboxylic acids arboxylic acids are organic acids: RCOOH or RCO2H Substituent Effects on Acidity  Useful ‘rule of thumb’ For weak acids: pH = pKa compound is approx. 50% ionised pH = pKa +1 compound is approx. 90% ionised pH = pKa +2 compound is approx. 99% ionised pH = pKa +3 compound is approx. 99.9% ionised pH = pKa +4 compound is approx. 99.99% ionised For weak bases: pH = pKa compound is approx. 50% ionised pH = pKa - 1 compound is approx. 90% ionised pH = pKa - 2 compound is approx. 99% ionised pH = pKa - 3 compound is approx. 99.9% ionised pH = pKa - 4 compound is approx. 99.99% ionised  Acidic Functional Groups dic groups are those that can donate (or lose) a proton. Key Features: 1. The presence of a hydrogen atom (proton) which can dissociate from the group. 2. The ability of the remaining atoms to DELOCALISE the resulting negative charge. Lower the pKa the stronger the acid Bases Here we are looking at the ability to accept a proton from another molecule. The base must have an unbonded electron pair with which the proton can bond. Consider ammonia in water: NH3 + H2O NH4 + OH Because it is accepting a proton from water it is acting as a base. We can measure base strength by using pKa values of the conjugate acids formed.  The larger the pKa of the conjugate acid, the stronger is the base is a stronger base thanCH COO CH3CH2O 3 So far then we have looked at a number of general structural considerations for organic molecules. We will see how these can influence the properties of various functional groups. Some properties, especially resonance and conjugation, are particularly important in aromatic molecules Remember also, functional groups allow drugs to combine with target molecules in the body Functional Groups groups are those that can accept (or gain) a proton. eatures: e presence of a lone pair of electrons which can bind to and accept a pro e availability of these electrons determines the basicity of the functional