Organic reactions 5 Connections Building on: Arriving at: Looking forward to: • Drawing molecules realistically ch2 • Why molecules generally don’t react • The rest of the chapters in this book • Ascertaining molecular structure with each other! spectroscopically ch3 • Why sometimes molecules do react • What determines molecular shape and with each other structure ch4 • In chemical reactions electrons move from full to empty orbitals • Molecular shape and structure determine reactivity • Representing the movement of electrons in reactions by curly arrows Chemical reactions Most molecules are at peace with themselves. Bottles of water, or acetone (propanone, Me2C=O), or methyl iodide (iodomethane CH3I) can be stored for years without any change in the chemical com- position of the molecules inside. Yet when we add chemical reagents, say, HCl to water, sodium cyanide (NaCN) to acetone, or sodium hydroxide to methyl iodide, chemical reactions occur. This chapter is an introduction to the reactivity of organic molecules: why they don’t and why they do react; how we can understand reactivity in terms of charges and orbitals and the movement of elec- trons; how we can represent the detailed movement of electrons—the mechanism of the reaction— by a special device called the curly arrow. To understand organic chemistry you must be familiar with two languages. One, which we have concentrated on so far, is the structure and representation of molecules. The second is the descrip- tion of the reaction mechanism in terms of curly arrows and that is what we are about to start. The first is static and the second dynamic. The creation of new molecules is the special concern of chem- istry and an interest in the mechanism of chemical reactions is the special concern of organic chem- istry. Molecules react because they move. They move internally—we have seen (Chapter 3) how the stretching and bending of bonds can be detected by infrared spectroscopy. Whole molecules move continuously in space, bumping into each other, into the walls of the vessel they are in, and into the solvent if they are in solution. When one bond in a single molecule stretches too much it may break Ǡ and a chemical reaction occurs. When two molecules bump into each other, they may combine with The activation energy, also the formation of a new bond, and a chemical reaction occurs. We are first going to think about colli- called the energy barrier for a sions between molecules. reaction, is the minimum energy molecules must have if they are Not all collisions between molecules lead to chemical change to react. A population of a given molecule in solution at room All organic molecules have an outer layer of many electrons, which occupy filled orbitals, bonding temperature has a range of and nonbonding. Charge–charge repulsion between these electrons ensures that all molecules repel energies. If the reaction is to each other. Reaction will occur only if the molecules are given enough energy (the activation energy occur, some at least must have for the reaction) for the molecules to pass the repulsion and get close enough to each other. If two an energy greater than the activation energy. We shall molecules lack the required activation energy, they will simply collide, each bouncing off the elec- discuss this concept in more trons on the surface of the other and exchanging energy as they do so, but remain chemically detail in Chapter 13. 114 5 . Organic reactions unchanged. This is rather like a collision in snooker or pool. Both balls are unchanged afterwards but are moving in different directions at new velocities. on collision course impact after collision Charge attraction brings molecules together In addition to this universal repulsive force, there are also important attractive forces between mole- cules if they are charged. Cations (+) and anions (–) attract each other electrostatically and this may be enough for reaction to occur. When an alkyl chloride, RCl, reacts with sodium iodide, NaI, in ace- í + tone (propanone, Me2C=O) solution a precipitate of sodium chloride forms. Sodium ions, Na , and We saw why these atoms form an ionic – compound in Chapter 4. chloride ions, Cl , ions in solution are attracted by their charges and combine to form a crystalline lattice of alternating cations and anions—the precipitate of crystalline sodium chloride. This inorganic style of attraction is rare in organic reactions. A electrostatic more common cause of organic reactions is attraction between a attraction charged reagent (cation or anion) and an organic compound that has a dipole. An example that we shall explore in this chapter is δ– O δ+ CN the reaction between sodium cyanide (a salt, NaCN) and a car- charged bonyl compound such as acetone. Sodium cyanide is made up reagent + – í of sodium cations, Na , and cyanide anions, CN , in solution. C=O dipole We analysed the orbitals of the Acetone has a carbonyl group, a C=O double bond, which is carbonyl group in Chapter 4 and established that the reason for the polarized because oxygen is more electronegative than carbon. polarity is the greater electronegativity The negative cyanide ion is attracted to the positive end of the electrostatic of the oxygen atom. attraction carbonyl group dipole. H It is not even necessary for the reagent to be charged. Ammonia δ– O δ+ N H also reacts with acetone and this time it is the lone pair of electrons H —a pair of electrons not involved in bonding and concentrated on the nitrogen atom of the uncharged ammonia molecule—that is lone pair C=O dipole of electrons attracted to the positive end of the carbonyl dipole. Polarity can arise from σ bonds too. The most electronegative element in the periodic table is flu- orine and three fluorine atoms on electropositive boron produce a partially positively charged boron atom by σ bond polarization. The negative end of the acetone dipole (the oxygen atom) is attracted to the boron atom in BF3. electrostatic F δ– attraction B–F dipole δ δ – FB+ δ– O δ+ FB F δ– C=O dipole But we have not told you the whole story about BF3. Boron is in group 3 and thus has only six electrons around it in its trivalent compounds. A molecule of BF3 is planar with an empty p orbital. This is the reverse of a lone pair. An empty orbital on an atom does not repel electron-rich areas of other molecules and so the oxygen atom of acetone is attracted electrostatically to the partial positive charge and one of the lone pairs on oxygen can form a bonding interaction with the empty orbital. We shall develop these ideas in the next section. Chemical reactions 115 So, to summarize, the presence of a dipole in a molecule represents an imbalance in the distribu- tion of the bonding electrons due to polarization of a σ bond or a π bond or to a pair of electrons or an empty orbital localized on one atom. When two molecules with complementary dipoles collide and together have the required activation energy to ensure that the collision is sufficiently energetic to overcome the general electronic repulsion, chemical change or reaction can occur. Orbital overlap brings molecules together Other organic reactions take place between completely uncharged molecules with no dipole moments. One of the old ‘tests’ for unsaturation was to treat the compound with bromine water. If the brown colour disappeared, the molecule was unsaturated. We don’t use ‘tests’ like these any more (spectroscopy means we don’t need to) but the reaction is still an important one. A simple symmetrical alkene combines with symmetrical bromine in a simple addition reaction. The only electrons that might be useful in the H kind of attraction we have discussed so far are the HH Br Br H lone pair electrons on bromine. But we know from + many experiments that electrons flow out of the Br H H H Br alkene towards the bromine atom in this reac- H tion—the reverse of what we should expect from electron distribution. The attraction between these molecules is not electrostatic. In fact, we know that reaction occurs because the bromine molecule has an empty orbital available to accept electrons. This is not a localized atomic orbital like that in the BF3 molecule. It is the antibonding orbital belonging to the Br–Br σ bond: the σ* orbital. There is therefore in this case an attractive interaction between a full orbital (the π bond) and an empty orbital (the σ* orbital of the Br–Br bond). The molecules are attracted to each other because this one interaction is between an empty and a full orbital and leads to bonding, unlike all the other repulsive interactions between filled orbitals. We shall develop this less obvious attraction as the chapter proceeds. Most organic reactions involve interactions between full and empty orbitals. Many also involve charge interactions, and some inorganic reactions involve nothing but charge attraction. Whatever í the attraction between organic molecules, reactions involve electrons moving from one place to Terms such as σ bond, σ∗ orbital, π another. We call the details of this process the mechanism of the reaction and we need to explain bond, π∗ orbital, lone pair, atomic and molecular orbital, and bonding and some technical terms before discussing this. antibonding orbital, are all explained in Chapter 4. Electron flow is the key to reactivity The vast majority of organic reactions are polar in nature. That is to say, electrons flow from one molecule to another as the reaction proceeds. The electron donor is called a nucleophile (nucleus- Ǡ loving) while the electron acceptor is called the electrophile (electron-loving).