Alkylhalides PDF

ALKYLHALIDES (Organohalogens)  HALOGENOALKANES ▪ Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine)  HALOGENOALKANES ▪ Halogenoalkanes fall into different classes depending on how the halog...

ALKYLHALIDES (Organohalogens)  HALOGENOALKANES ▪ Halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine)  HALOGENOALKANES ▪ Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms ▪ There exist some chemical differences between the various types ▪ Primary (1°) halogenoalkane: the carbon which carries the halogen atom is only attached to one other alkyl group ▪ Secondary (2°) halogenoalkane: the carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different ▪ Tertiary (3°) halogenoalkane: the carbon atom holding the halogen is attached directly to three alkyl groups, which may be any combination of same or different  NOMENCLATURE  To name a halogenoalkane: 1. Name the longest consecutive carbon chain 2. Carbon atoms bonded to halogen atoms are given the lowest possible numbers 3. Halogens are named before alkyl substituents : fluorine atom is named as fluoro chlorine atom is named as chloro bromine atom is named as bromo iodine atom is named as iodo 4. For more than one of the same halogen: di = 2; tri = 3; tetra = 4 5. If there are more than one type of halogen, name them alphabetically i.e. bromo is named before chloro Exercise: Name the following molecules (a) (b) (c) (d) ▪ Solution (a) 1-chloro-4-iodo-2-methylpentane (b) 2-bromo-1,2-dichlorobutane (c) 1-chloro-2-fluoro-2,3-dimethylbutane (d) 2-chloro-3-ethylpentane PHYSICAL PROPERTIES OF HALOGENOALKANES Boiling Point  The van der Waals’ Dispersion forces get stronger as the molecules get longer and have more electrons.  Due to an increase in the sizes of the temporary dipoles that are set up. Therefore the boiling points increase as the number of carbon atoms in the chains increases  The increase in boiling point as you go from a chloride to a bromide to an iodide (for a given number of carbon atoms) is also because of the increase in number of electrons leading to larger dispersion forces  The carbon-halogen bonds are generally are polar, because the electron pair is pulled closer to the halogen atom than the carbon. ▪ The dipole-dipole attractions will decrease as the bonds get less polar (as you go from chloride to bromide to iodide). ▪ However, there is still an increase in boiling points. ▪ This shows that the effect of the permanent dipole-dipole attractions is much less important than that of the temporary dipoles which cause the dispersion forces ▪ The large increase in number of electrons by the time you get to the iodide completely outweighs the loss of any permanent dipoles in the molecules PHYSICAL PROPERTIES OF HALOGENOALKANES  The boiling points decrease as the isomers go from a primary to a secondary to a tertiary halogenoalkane. This is as a result of the decrease in the effectiveness of the dispersion forces  The temporary dipoles are greatest for the longest molecule. The attractions are also stronger if the molecules can lie closely together  The tertiary halogenoalkane is very short and fat, and will not have much close contact with its neighbours  PHYSICAL PROPERTIES OF HALOGENOALKANES Solubility in water  The halogenoalkanes are at best only very slightly soluble in water  In order for a halogenoalkane to dissolve in water you have to break attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and break the hydrogen bonds between water molecules.  Both of which requires a lot of energy  Energy is released when new attractions are set up between the halogenoalkane and the water molecules. These will only be dispersion forces and dipole-dipole interactions.  These are not as strong as the original hydrogen bonds in the water, and so not as much energy is released as was used to separate the water molecules  The energetics of the change are sufficiently "unprofitable" that very little dissolves CHEMICAL REACTIVITY OF HALOGENOALKANES  In order for anything to react with the halogenoalkanes, the carbon- halogen bond must be broken  Because the breaking of the carbon-halogen bond gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order  Of the four halogens, fluorine is the most electronegative and iodine the least. That means that the electron pair in the carbon-fluoride bond will be dragged most towards the halogen end CHEMICAL REACTIVITY OF HALOGENOALKANES  The electronegativities of carbon and iodine are equal and so there will be no separation of charge on the bond  The strengths of various bonds (all values in kJ mol-1) are as follows: C-H 413 C-F 467 C-Cl 346 C-Br 290 C-I 228  The bonds get weaker as you go from chlorine to bromine to iodine. Therefore the rate of reaction is as follows: RCl < RBr < RI CHEMICAL REACTIVITY OF HALOGENOALKANES  Iodoalkanes are the most reactive and fluoroalkanes are the least  Fluoroalkanes are very unreactive and does not participate in chemical reactions  SUBSTITUTION REACTION  One of the important set of reactions of halogenoalkanes involves replacing the halogen with something else - substitution reactions  SUBSTITUTION REACTION  Substitution reactions involve either: (a) the carbon-halogen bond breaking to give positive and negative ions. The ion with the positively charged carbon atom then reacts with species that has a fully or slightly negative charge (b) Something with either a fully or slightly negative charge is attracted to the slightly positive carbon atom and weakens the carbon-halogen bond  Neither of the two mechanism will work for fluoroalkanes simply because it requires too much energy to break the carbon-fluorine bond  NUCLEOPHILIC SUBSTITUTION ▪ A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else ▪ Nucleophiles are either fully negative ions, or else have a slightly negative charge somewhere on a molecule ▪ Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia SN2 MECHANISM ▪ Nucleophilic substitution by SN2 mechanism takes place fastest in primary halogenoalkanes ▪ SN2 reaction: S - substitution, N - nucleophilic, and the 2 is because the initial stage of the reaction involves two species – the halogenoalkane and the nucleophile ion (Nu- ) Summary SN2 MECHANISM ▪ The lone pair on the Nu- ion will be strongly attracted to the positive carbon, and will move towards it, beginning to make a co-ordinate (dative covalent) bond. ▪ In the process the electrons in the C-Br bond will be pushed even closer towards the bromine, making it increasingly negative. ▪ The movement goes on until the -Nu is firmly attached to the carbon, and the bromine has been expelled as a Br- ion ▪ The Nu- ion approaches the positive carbon from the side away from the bromine atom. ▪ The large bromine atom hinders attack from its side because the bromide is negative, it would repel the incoming Nu-. ▪ This attack from the back is important in the SN2 mechanism. ▪ There is a point in which the Nu- is half attached to the carbon, and the C-Br bond is half way to being broken. This is called a transition state … ▪ It is not an intermediate because it cannot be isolated even for a very short time. It is the mid-point of a smooth attack by one group and the departure of another  SN1 MECHANISM ▪ In tertiary halogenoalkanes, the nucleophilic substitution occurs via a SN1 mechanism ▪ The reaction happens in two stages. In the first, a small proportion of the halogenoalkane ionizes to give a carbocation and a bromide ion ▪ This reaction is possible because tertiary carbocations are relatively stable compared with secondary or primary ones. ▪ The rate of reaction is governed by how fast the halogenoalkane ionizes  SN1 MECHANISM ▪ Once the carbocation is formed, it reacts immediately with a Nu-. The lone pair on the nucleophile is strongly attracted towards the positive carbon, and moves towards it to create a new bond ▪ Because this initial slow step only involves one species, the mechanism is described as SN1 - substitution, nucleophilic, one species taking part in the initial slow step ▪ Primary halogenoalkane will not react via SN1 mechanism because the carbocation that would be formed is unstable NUCLEOPHILIC SUBSTITUTION IN SECONDARY HALOGENOALKANES  Secondary halogenoalkanes will use both mechanisms - some molecules will react using the SN2 mechanism and others the SN1.  The SN2 mechanism is possible because the back of the molecule is not completely cluttered by alkyl groups and so the approaching nucleophile can still get at the positive carbon atom. ▪ The SN1 mechanism is also possible because the secondary carbocation formed in the slow step is more stable than a primary one. ▪ However the carbocation is not as stable as a tertiary one, therefore the SN1 route is not as effective as it is with tertiary halogenoalkanes NUCLEOPHILIC SUBSTITUTION IN THE ARYL HALIDES ▪ Simple aryl halides like chlorobenzene are very resistant to nucleophilic substitution. ▪ The carbon-chlorine bond in chlorobenzene is very strong due to the interaction between one of the lone pairs on the chlorine atom and the delocalised ring electrons, and this strengthens the bond. ▪ Both of the mechanisms for substitution reaction involve breaking the carbon-halogen bond at some stage. ▪ The more difficult it is to break, the slower the reaction will be.  DEHYDROHALOGENATION: REMOVAL OF HALOGEN FROM HALOGENOALKANES E.g. The elimination reaction involving 2-bromopropane and hydroxide ions ▪ 2-bromopropane is heated under reflux with a concentrated solution of sodium or potassium hydroxide in ethanol ▪ (Heating under reflux involves heating with a condenser placed vertically in the flask to avoid loss of volatile liquids) ▪ Propene is formed and, because this is a gas, it passes through the condenser and can be collected Set Up for Reflux  DEHYDROHALOGENATION ▪ Dehydrohalogenation is the removal of hydrogen halides from halogenoalkanes. ▪ In elimination reactions, the hydroxide ion acts as a base - removing a hydrogen as a hydrogen ion from the carbon atom next door to the one holding the bromine. ▪ The resulting re-arrangement of the electrons expels the bromine as a bromide ion and produces an alkene. E2 MECHANISM