Organic Functional Groups Lecture 2 PDF

ORGANIC FUNCTIONAL GROUPS Dr Graeme Kay Academic Strategic Lead – Chemical Sciences 01224 262548 [email protected] Lecture 2 IONIC STRUCTURES AND FORMAL CHARGE So far we have seen the formation of ions in ionic bonding (i.e. loss and gain of an electron). We must also realise that ioni...

ORGANIC FUNCTIONAL GROUPS Dr Graeme Kay Academic Strategic Lead – Chemical Sciences 01224 262548 [email protected] Lecture 2 IONIC STRUCTURES AND FORMAL CHARGE So far we have seen the formation of ions in ionic bonding (i.e. loss and gain of an electron). We must also realise that ionic structures can also exist within molecules. Hence we are looking at cations and anions within a molecule. This molecule will have an overall formal charge associated with it. If we look at the structures of ammonia and water we see that they are neutral molecules: H N H H O H H However, if we look at ammonium and hydronium cations they are charged: H H H N H H O H H Why is it charged? Why is it charged? H H N H H Why is ammonia neutral? H N H H For each H atom: number of valence electrons = 1 Each H atom is sharing a pair of electrons, so we assign 1 to H Formal charge for each H = 1 – 1 = 0 For the N atom: number of valence electrons = 5 It is bonded to 3 H atoms (3 pairs electrons), so we assign 3 electrons to the N It has 2 unbonded electrons, so we assign these 2 to the N Number of electrons assigned to N = 3 + 2 = 5 Formal charge for N = 5 – 5 = 0 So the overall charge is 0 In organic molecules ionic structures involving the C atom should be recognised. When we have a positively charged C ion this is called a carbocation C When we have a negatively charged C ion this is called a carbanion. C Resonance If we draw the Lewis structure for the carbonate ion CO32─ , for example, we have a problem! We could draw Which one is correct? Let’s realise that they are all equivalent to eachother. Let us appreciate 2 important things: 1. Each atom has its octet of electrons. 2. We can convert one structure into any other by changing only the position of the electrons. We can show the movement of a pair of electrons using a curly or curved arrow: Movement of a pair of electron O O So with the carbonate ion becomes we can show C C O O O O We can also see that: O O becomes C C O O O O Although C─O bond lengths are different to those of C=O bonds, the length of all carbon – oxygen bonds in the carbonate ion are equal. Therefore, none of the structures we have drawn can be correct! The answer to our problem is called resonance theory. When a molecule or ion can be represented by Lewis structures that differ only in the position of their electrons: 1. No structure, or resonance structure, actually exists 2. The actual structure is a hybrid of each resonance structure. To show resonance structures we use a double- So with the carbonate ion we could draw: So the hybrid structure is Resonance structures are very important when considering the stability of molecules. The delocalisation of electrons we see in molecules that show resonance stabilises the molecule. This stabilising effect means the molecule has lower energy The difference between the actual energy and expected energy is called the Resonance Energy of the molecule. The more resonance structures we can draw, the greater the extent of delocalisation.