Block 7 alkenes and aromatics part 2.

Annotations:

• Alkenes readily react with hydrogen in the presence of a metal catalyst such as platinum, paladium or nickel. They are an example of reduction reactions, the net result being loss of unsaturation. Complex reaction mechanisms. IN syn addition H2 is bound to the metal surface of the catalyst as two H atoms. The are both added across the double bond, both on the same face of the alkene or alkyne.

1. Alkyne hydrogenation

   Annotations:

   • Product is usually the alkane rather than the alkene. This is because when the normal metal catalysts are used the alkene produced is further reduced into the alkane.
   1. Lindlar's catalyst

      Annotations:

      • Herbert Lindlar invented catalyst palladium immoblized on a solid support of barium sulfate which is poisoned by adding a small amount of quinoline. This slows the reaction down so the alkene can be isolated before it becomes an alkane. There are other poisoned catalysts.
2. Catalysts platinum, palladium, nickel

Annotations:

• Aromatics can be isolated from natural sources such as plants, from coal tar and oil and often have distinctive odours.

1. structure of benzene

   Annotations:

   • Benzene is the parent compound of a whole class of compounds called aromatics. It is a six carbon ring with alternating double bonds with the formula C6H6.
   1. predicting rings/double bonds from formula

      Annotations:

      • For every two hydrogen that are removed from a saturated acyclic formula a double bond or ring must be added.
      1. 200 structural isomers from C6H6

         Annotations:

         • Not all are viable as they are too strained but some can be synthesised.
   2. Bond lengths

      Annotations:

      • IN ethene Double bond length is 134 pm Single bond length is 146 pm Therefore benzene should have more asymetrical shape than is usually drawn. It doesn't because of oscillation between postion of single and double bonds.
      1. Double bonds not fixed

         Annotations:

         • Only one product is produced form a substitution reaction with an electrophile. So model of asymetrical benzene is modified. Double bonds oscillate between different positions which allows for identical hydrogen environments and the one product.
         1. Kekule's hypothesis

            Annotations:

            • The consequences of this for dichlorobenzene are that if the bonds oscillate then there are only 3 structural isomers. If the bonds are fixed there are 4. Experimentally there are 3 so the hypothesis seems to be correct.
            1. X-ray Chrystallography

               Annotations:

               • Showed that all the bonds are equal at 140 pm
      2. benzene bond lengths

         Annotations:

         • Benzene is a perfect hexagon with bond lengths of 140 pm long. It is also planar. The bond length lies between the c-c bond of 154 pm and the c=c bond of 133 pm. All bond angles are 120 degrees and C-H bonds are all 108 pm.
      3. Resonance

         Annotations:

         • As benzene has pi bonds it can exhibit resonance and two possible structures can be drawn. The two resonance forms contribute equally to the benzene structure, each bond is intermediate between the two forms and the electrons are delocalised around the ring.
   3. conjugated molecules

      Annotations:

      • Conjugated molecules in organic chemistry have alternating single and double bonds.
      1. benzene conjugated

         Annotations:

         • In spite of the explanation fo resonance to explain benzene's shape it must be remembered that benzene has a strong tendncy to maintain an intact conjugated  6 pi electron system with 3 double bonds.
      2. conjugation in organic molecules

         Annotations:

         • conjugation in organic molecules allows electrons to be shared over a group of atoms - to become delocalised.
   4. Bonding in benzene

      Annotations:

      • Resonance structures and electron delocalisation can also be represented in terms of orbitals. With 3 bonds each carbon is sp2 hybridised. The molecules is perfect for a planar sigma bond skeleton. THe remaining pi orbitals are perpendicular above and below the plane of the ring. Each orbital has a single pi electron associated with it. It can react strongly with either neighbour. The result is a delocalised pi electron system above and below the pane fo the benzene ring.
      1. Circular Pi bond

         Annotations:

         • The delocalised pi electron system forms circular orbitals above and below the benzene ring plane. It forms 6 molecular orbitals. 3 bonding and 3 antibonding. IN an energy level diagram the three lower energy bonding orbitals are each filled with a pair of electrons.
         1. molecular orbitals

            Annotations:

            • benzene has 3 bonding MOs and 3 antibonding electrons.(one electron from each of the singly occupied p orbitals) The 3 bonding MOs are filled with an electron pair each.
            1. bonding MOs
               1. antibonding MOs

                  Annotations:

                  • THe higher energy antibonding MOs have progressively more nodalplanes dissecting the ring and they are out of phase.
   5. Stability of benzene

      Annotations:

      • Benzene has very different reaction patterns to other unsaturated systems such as alkenes or alkynes. It does not undergo addition reactions easily. This is because of benzenes tendancy to maintain a  strong 6 pi electron system of alternating double bonds. This is because it is energetically favourable to maintain this 6 pi electron system
   6. distinctive chemistry of aromatics

      Annotations:

      • The reaction of aromatic compounds with electrophilic agents to give disubstituted compounds does not readily occur. The reaction of aromatics  is distinct from the reactions with alkenes. Reactions proceed by a substitution machanism.
      1. electrophilic aromatic substitution

         Annotations:

         • Electrophile is the reagent and nucleophile is provided by the double bond. Electrophile interacts with pi electrons at double bond of ring to produce intermediate species carbocation. The main task is to first identify the electrophile in each of these reactions though the general mechanism is the same.
         1. Whetland intermediate or sigma complex

            Annotations:

            • resonance can be used to describe the distribution of charge around the ring. The electron delocalisation can be drawn as a broken line around the inside of the ring. However this broken line stops before the site of single bond where the substitution reaction is occurring. (C1 is already connected to other atoms by single bonds)
         2. Nitration reactions

            Annotations:

            • Aromatic compounds treated with a nitrating mixture -  a mixture of concentrated nitric and sulfuric acids. Nitrated aromatics are formed. Sulfuric acid is stronger than nitric acid so it protonates HNO3 to give H2NO3 which can lose water to give the nitronium ion NO2+. The resulting carbocation must lose a proton to gain aromaticity. several species from nitrating mixture could do this eg. H2O HSO4- or NO3-. They are represented by B.
         3. sulfonation reactions

            Annotations:

            • Aromatics are treated with hot concentrated sulfuric acid alone to produce aromatic sulfonic acids and water. (R-SO3H) The electrophile is sulfur trioxide SO3(it is not a cation but polarised so decent electrophile The reaction mechanism is similar to nitration but the steps are reversable. But as one of the products is water. Removing water can shift equilibria to product side.
            1. Desulfonation

               Annotations:

               • Desulfonation can occur if more water is added to the reaction mix just as removing water helps sulfonation. THe reaction is an equilibria.
         4. halogenation reactions

            Annotations:

            • Whilst halogens are polarised by double bonds in alkenes and react readily this is not so with aromatics. A Lewis acid catalyst is needed.
            1. Lewis acid catalyst

               Annotations:

               • A lewis acid is a species that can accept an electron pair from a lewis base to form a lewis adduct. Metal chlorides such as tin tetrachloride, iron or aluminium chlorides all behave as an acceptor of a non bonded pair of electrons from the Chlorine atoms. The resulting polasied chlorine atoms are activated to act as electrophiles in a halogenation reaction.
            2. works for chlorine and bromine
            3. does not work for Fluorine or iodine
      2. substitution favoured over addition

         Annotations:

         • mechanistically the first step in both types of reaction is the formation of a cyclic bromonium ion. The activation energy is the same in both cases. The second step is different. For the addition reaction of the alkene, the bromonium ion is ring opend by the incoming bromide ion. For the hypothetical substitution reaction with the alkene a proton would be lost and the alkene would reform with a bromine substituent rather than hydrogen. For the hypothetical addition reaction with the aromatic the second step is endothermic and so unfavourable. The formation of one product from two reactants also means there is a decrease in entropy so also thermodynamically unfavourable.
         1. addition reactions non spontaneous for aromatics
      3. Freidel-crafts reactions

         Annotations:

         • When planning synthesis of a complex organic molecule one of the main concerns is the building of the correct carbon framework. Freidel-craft alkylation reactions are concerned with adding carbon chains to aromatic rings. They are divided into alkylation and acylation.
         1. Acylation

            Annotations:

            • Used to solve problems mentioned in alkylation. Acyl groups (RCOR) are added to an aromatic ring. The electrophile is generated by the reaction of the acyl chloride and the AlCl3 to produce the electrophilic species of the acylonium ion. THe acytlonium ion is stablised by resonance with two resonance forms possible. THese reactions are quite typical electrophilic aromatic substution reactions.
            1. Synthesis

               Annotations:

               • Acylonium ions do not rearrange and so do not produce a product mix. Furthermore acylonium ions deactivate the aromatic ring to prevent further substitution. Acyl groups withdraw electron density form the ring and make them less attractive to electrophiles. Further treatment with zinc amalgam produces the corresponding alkylated product. This is called Clemmensen reduction.
         2. Alkylation

            Annotations:

            • Concerned with adding carbon chains to aromatic rings. Normally a Clhoroalkane and AlCl3 are used to generate the carbocation. ON industrial scale zeolite is used to avoid generating hazardous waste from metal halides. One plrblem is with carbocations arranging to a more stable carbocation - from primary to tertiary. This may happen via a hydride or alkyl group shift. Another problem is that substituted rings are more reactive than a benzene so the product may be more reactive than the reactant resulting in further reactions and a mix of final products.
            1. synthesis
      4. Substitution and substituents

         Annotations:

         • ortho, meta and para describe where substuents are in relation to each other. they also correspond to numbers so 1,2 dichlorobenzene is ortho, 1,3 is meta and 1,4 is para. This can lead to 3 positional isomers of the product. Statistically you might expect that the distribution of these iosomers would be 40/40/20 (there is one para but two ortho and meta positions) but this is not observed due to directing effects.
         1. Directing effects

            Annotations:

            • When a second subtituent is introduced the first exerts a directing effect on it. Types and amounts of products of adding a second substituent can be predicted based on the common mechanism for electrophilic substitution. The formation of the carbocation intermeddiate is the rate determining step. The carbocation can be stablised by an input of electron density to reduce the overall positive charge and by resonance. The more stable the carbocation the more likely it is to form, resulting in certain products predominating.
         2. activating groups

            Annotations:

            • Some substituents increase the stability of the carbocation by having an electron density donating effect to the pi system. The more electron rich the ring, the more attractive it is to incoming electrophiles, the faster the rate, of reaction relative to the rate of benzene. The ring is said to be activated. Activators include O, R, OR, NH2. Small difference in electronegativity on C and H leads to a slight build up of electron density on the C in the CH bond. The electron density on the C means that the alkyl group acts as a weak electron donating group. This is known as a positive inducting effect. In addition one of the HC bonds is perpendicular to the ring and an overlap of orbitals can occur resulting in further electron donation. Therefore aromatic rings with alkyl groups are more reactive as they have more electron density available.
            1. Alkyl Substituents

               Annotations:

               • Alkyl substituents: 1. Donate electron density to the aromatic ring causing a positive inductive effect (+l). 2. Activate the ring towards electrophiles - the ring is more reactive towards other alkyl groups. 3. Direct substitution towards the ortho and para positions.
            2. oxygen and nitrogen

               Annotations:

               • Aomino groups, Alkoxy(OR) and hydroxy groups are deactivating. As O and N are electronegative groups you would expect them to withdraw electron density form the ring (polarising the C-O bond so that C is more positive. TRhis is called a negative inductive effect(-l) However the non bonded pairs on the O or N interact with the pi system and increase electron density. This is called a mesomeric effect (+m). Additional resonance structures are possible for the ortho and para positions but not for the meta position because the non bonded pair do not interact with the pi system. This is reflected in observed products. So these groups activate the ring towards electrophiles and direct twoards the ortho and para position
         3. deactivating groups

            Annotations:

            • Some substituents withdraw electron density form the ring causing it to be electron poor. They have a deactivating effect as reaction rates will then be slower. These groups are NO2 CO2R CL and CN. IN the chloronation of nitrobenzene meta predominates because all the charges remain separate from the positive charge on the nitrogen atom. So these groups have a -l and a -m. They deactivate the ring to electrophiles and they  dorect substitution to the meta position.
            1. deactivating halogens

               Annotations:

               • Halogen substituents are also deactivating towards electrophiles - a negative inductive effect. They direct the ring to the ortho and para positions which is a weak mesomeric effect (+m). The carbon -halogen bond is polarised. The non -bonding pairs conjugate on the halogen with the Pi system. The electron withdrawing effect is great enough to destablize the ring  but the additional resonance forms that arise direct to the ortho and par positions over the meta position.
         4. Order of substitution

            Annotations:

            • Order of substitution is important because the first substituent has a directing effect on the second one and influences the position of the second substituent. Thus influencing the major product. It also effect the reactivity of the ring.
      5. Diazonium salts

         Annotations:

         • Important for Dyes. Contain the FG RN-triple-bond-N Important intermediates in the synthesis of organic compounds.
         1. Synthesis of Aniline

            Annotations:

            • Aniline is an aminobenzine that was originally prepared by destructive distillation of natural indigo form the indigo plant. But later replaced by synthesis form coal tar. Benzene is nitrated using nitric and sulfuric acids. It is then reduced by hydrogen generated by dilute HCl or iron or tin to form the NH3 group instead of (NO2). Aniline can then be used to synthesise several dyes. Like aniline yellow.
         2. Reactions of diazonium
            1. Substitution
            2. Coupling

               Annotations:

               • Coupling reactions provide a means of producing azo dyes. Diazonium salts are themselves reagents and take part in coupling reactions with other aromatic compounds via the two nitrogen atoms. products can be azo dyes.
2. Aromaticity

   Annotations:

   • Aromaticity does not just mean scent, some aromatics have no scent. It also referrs to the electron properties of this class, in particular how they get their stability and their patterns of reactivity.
   1. Aromaticity

      Annotations:

      • refers to the specific electron properties of planar cyclic conjugated systems such as benzene.
      1. Huckle's rule

         Annotations:

         • Compounds that are a single ring contain (4n+2) Pi electrons where n is an integer. Such compounds containing this number of pi electrons benefit from more stability than those that don't. This extra aromatic stablization is about 260 KJ mol-1
   2. 8 ring

      Annotations:

      • Not aromatic as it does not meet the 4n+2 rule and is not planar so should undergo addition reactions more easily.
   3. pyrrole

      Annotations:

      • 4 carbon and one nitrogen ring. Is planar and has two double bonds Has a nitrogen atom in the ring the non bonded pair can participate in the pi system. making 6 electrons available so it is aromatic.
   4. ions

      Annotations:

      • Some ions can be aromatic when the compounds they are derived from are not. The increase or decrease in the number of electrons can allow the ion to fulfill the 4n+2 rule.
   5. energetically favourable

      Annotations:

      • As aromaticity confers stability it is energetically favourable to be aromatic, to obtain this 6 electron configuration.
3. definition

   Annotations:

   • For a single ring system to be classed as aromatic it must be cyclic, conjugated and planar. It must also contain (4n+2) Pi electrons. (it must be planar so that the orbitals can overlap fully.) Aromacity can also be achieved if other electrons in the system can participate.