Hidrocacbon

21.1. INTRODUCTION In chapters 21 and 22 we shall look at the reactions of different types of organic molecule. We shall attempt to predict main reaction types from structure and then, for each type of molecule, we shall briefly summarise reactions which do not easily fall into the 12 types described in the last chapter. If you are unfamiliar with the types of molecule considered in these chapters, then chapter 27 (Nomenclature) should help. Look up each type of molecule as you consider its reactions. Some indication of reaction conditions will be given. It is ridiculous to learn a whole list of conditions: if they are needed for laboratory procedures they can be looked up. However, reaction conditions also give an indication of the ease with which a reaction occurs. They should certainly be absorbed at a sub-conscious level to help you acquire a feel for relative reactivities. Once you have such a feel, you will be able to predict reaction conditions as accurately as can reasonably be expected. If your examiners require more, they are wasting your time. 21.2. ALKANES 21.2.1. Predictions: Alkanes have no regions of either exposed nuclear charge or high electron density and are therefore unaffected by either nucleophiles or electrophiles. Moreover, there are no polarised bonds, so reactions occur homolytically, when they occur at all. In addition, bonds are strong so reactive free radicals are needed to make alkanes react. Finally, there are no multiple bonds, so addition is not possible. Nor is elimination favoured because this would involve simultaneous attack on hydrogen atoms attached to two adjacent carbon atoms - an unlikely event. The result of attack by free radicals on an alkane is therefore substitution i.e. the nett reaction is homolytic substitution, via mechanism 1 (FIG. 20.1.). 21.2.2. Homolytic substitution in alkanes: examples of attacking free radicals: 21.2.3. Other reactions: Two other homolytic reactions undergone by alkanes are cracking and combustion. These are not chain reactions, but like homolytic substitution, the conditions needed for reaction are extreme, i.e. high temperature: i) cracking: The bonds break rather randomly in cracking reactions, producing a mixture of saturated and unsaturated hydrocarbons. .400 - 700°C 2CH 3 CH 2 CH 3 (g) .g . CH 4 (g) + CH 2 =CH 2 (g) + CH 3 CH=CH 2 (g) + H 2 (g) .C-C bond fission dehydrogenation Table 21.1. Examples of free radical substitution in alkanes (see section 20.3 for mechanism) Radical Reagent Conditions Product(s) lCl chlorine gaseous and UV light or in CCl 4 chloroalkane, dichloroalkanes etc lBr bromine gaseous/heat/UV bromoalkane, dibromoalkanes etc lSO 2 .OH fuming sulphuric acid heat alkanesulphonic acid (salts = detergents) lNO 2 concentrated nitric acid heat/gas phase nitroalkanes (mixture due to fission etc.) ii) combustion: CH 4 (g) + 2O 2 (g) g CO 2 (g) + 2H 2 O(g) 2C 2 H 6 (g) + 7O 2 (g) g 4CO 2 (g) + 6H 2 O(g) Note that combustion is an oxidation reaction. Alkanes may also undergo "autoxidation", by a free radical chain mechanism. This can be initiated by light, or an "initiator". Typical initiators are substances which produce free radicals, sometimes at higher temperatures or in the presence of light. Autoxidation is a bad term because it implies that the process takes place in the absence of any other reactant. In fact, the oxidising agent is atmospheric oxygen. 21.3. ALKENES 21.3.1. Predictions i) The double bond in alkenes is a region of high electron density which therefore attracts electrophiles. Moreover, the molecule is unsaturated and the attack results in addition. i.e. nett reaction is electrophilic addition (by mechanisms 8 and 9). However, the implied connection between unsaturation and addition begs the question, "What favours saturation over unsaturation?" The answer is illustrated in the equation below. The electrons involved in the bonds resulting from addition (bonds iii, iv and v), are held more tightly than they were before addition occured (bonds i and ii); electrons are pulled away from the double bond and the Br-Br bond, into single bonds where they are held more tightly. The relative tightness with which the electrons are held before and after addition can be understood in the following way. The pair of electrons in the p orbitals of double bond (i) are not particularly close to the two carbon nuclei. They become more strongly held in the two s bonds (iii and v) since these are directly inbetween the carbon and bromine nuclei. Moreover, the two electrons in bond (ii) between two large bromine atoms, become more tightly held in bonds (iii) and (v), where the smaller size of the carbon atoms makes the bonding electrons closer to the nuclei. The changes are reflected by (not explained by) the energy changes involved in the reaction. 21.3.2. Electrophilic addition to alkenes: examples of attacking electrophiles Table 21.2. Examples of electrophilic addition to alkenes. Electrophile reagent conditions main product(s) d+ d- *Bromine tetrachloromethane, r.t. -CHBr-CHBr- *Br-Br †Br-OH †Bromine in water, room temp. Water -CHBr-CHOH- plus some-CHBr-CHBr- The observable disappearance of brown colour makes this reaction useful as a test for double bonds. **H-Hal H-Hal gaseous -CH 2 -CHHal- H-OH acid catalysed room temp., some alkenes Industrially high temp and press used. E.g. ethanol using silcon dioxide coated w. H 3 PO 4 as catalyst (see also method below) -CH 2 -CHOH- H-OSO 3 H conc. sulphuric acid room temperature -CH 2 -CHOSO 3 H- Boiling the product with water gives alcohols by nucleophilic substitution, an important industrial process. 85% sulphuric acid at 0°C is used for the addition stage. *Or chlorine (faster than bromine) or iodine (v.v.slow). (Fluorine reacts differently and explosively with ethene to give carbon and HF gas.) †Or chlorine, or iodine. ** reaction rate: HF << HCl < HBr < HI (Can you explain?) 21.3.3. Predictions ii) The above reactions occur in the absence of conditions which produce free radicals. In conditions where free radicals are present (see section 20.12.1.) addition may occur via a homolytic mechanism, i.e. homolytic addition via mechanism 6 (FIG. 20.1.). Reactants which may add homolytically include: *Br-H (not HCl or HI), RS*-H, Cl 3 C*-Cl, and Cl 3 C*-H. * indicates the part of the molecule which forms the initial attacking radical. Polymerisation by a homolytic addition mechanism has already been discussed in section 20.12.2. 21.3.4. Another example of homolytic addition, but not a chain reaction, is the reduction of alkenes by hydrogen gas in the presence of a metal catalyst such as platinum, or finely powdered nickel. .Pt/200°C -CH=CH- + H 2 g -CH 2 -CH 2 - fine Ni/r.t. The binding sites on the nickel for hydrogen atoms are slightly further apart than the length of the H-H bond. This tends to split the hydrogen into reactive atoms. Hydrogenation of double bonds is an important process in the manufacture of some margarines. Saturated fats tend to be more solid than unsaturated oils, though the health implications are well known. 21.3.5. Other reactions: i) Oxidation: a) Combustion: CH 2 =CH 2 (g) + 3O 2 (g) g 2CO 2 (g) + 2H 2 O(g) b) With acidified potassium manganate (VII) solution: cold dil. -CH=CH- + H 2 O + [O] g -CH-CH- KMnO 4 . | | OH .OH Note that this reaction involves a readily observable change. The purple colour of the manganate (VII) disappears, and brown manganese (IV) oxide is precipitated. c) With ozone: O-O .\ . / .e.g. CCl 4 \ / \ / C=C + O 3 g . C C ./ . \ solution / .\ ./. \ O .an ozonide Ozonides are explosive and are not isolated. However, hydrolysis of the ozonide is a useful reaction. It produces carbonyl compounds (provided a reducing agent such as zinc dust and ethanoic acid is present to prevent oxidation of the carbonyls by the hydrogen peroxide): .O-O H 2 O .\ / . \ / r.t./warm \ / C C .g C=O + O=C + H 2 O 2 ./. \ . /. \ .Zn/HEt / . \ O The overall reaction with ozone, followed by hydrolysis, is known as ozonolysis and its usefulness lies in its power as an analytical tool: analysis of the resultant carbonyls gives information about the structure of the parent alkene. For example, what alkene would produce a mixture of propanone and ethanal on ozonolysis? 21.4. ALKYNES 21.4.1. Predictions i) The arguments are similar to those for alkenes. The triple bond in alkynes is a region of high electron density which therefore attracts electrophiles. Moreover, alkynes are unsaturated and attack results in addition. The nett reaction is therefore electrophilic addition via mechanisms 8 and 9 (FIG. 20.1.). Note that, as discussed in section 20.15, alkynes are often less reactive than alkenes to electrophiles, despite their higher electron density. Note also, that after addition to a triple bond, there is still a double bond. This may undergo further electrophilic addition. However, reactivity may be less than expected if the first addition to the triple bond has introduced, say, a halogen atom into the molecule: A halogen atom attached to a doubly bonded carbon atom has a negative inductive effect (section 21.5.). This reduces the electron density in the double bond and makes it less susceptible to electrophilic attack than a double bond in a simple alkene. Moreover, further addition will be directed as predicted by Markownikoff's rule (section 20.14.1.). 21.4.2. Examples of electrophilic addition to alkynes The electrophiles which add to alkynes are largely the same as those which add to alkenes (table 21.2.), and in the absence of free radicals, the main product is predicted by Markownikoff's rule. However, remember that alkynes are generally less reactive than alkenes and: (i) Bromine water does not react. (ii) the addition of halogens or halogen halides requires a halogen carrier catalyst such as FeBr 3 . Alternatively, UV light enables the reaction to proceed via a homolytic mechanism. However, under these conditions, the reaction with chlorine may be explosive, producing carbon and hydrogen chloride. (iii) addition of water under acid conditions requires mercury (II) sulphate to further catlyse the process. The method used is bubbling the alkene into hot dilute sulphuric acid containing the catalyst. The "enol" so produced is unstable and rapidly undergoes rearrangement to form a carbonyl compound. For example: The reaction is useful in the synthesis of a large range of organic compounds, especially when it is considered that carbon itself may be the starting point, via calcium(II) dicarbide! 2000°C CaO(s) + 3C(s) .g CaC 2 (s) + CO(g) CaC 2 (s) + 2H 2 O(l) .g Ca(OH) 2 (s) + CH=CH(g) 21.4.3. Predictions ii) Apart from electrophilic addition there is another fascinating property of alkynes. Electrons in an sp 1 orbital are closer to the nucleus than those in an sp 2 orbital, and even closer than those in an sp 3 orbital. Under certain conditions, a hydrogen next to a triple bond can actually be removed as a proton and the C-H bonding electron pair accomodated in the carbon atom's sp 1 orbital. Thus a carbanion is formed and the alkyne can be regarded as having slight acidic properties (section 21.4.4.). 21.4.4. Acidic properties. The acidic properties are shown in two ways: i) The amide ion is a strong enough base to remove the acidic hydrogen. The reagent is sodium dissolved in liquid ammonia. 2NH 3 (l) + 2Na(s) .g 2Na + - :NH 2 (am) + H 2 (g) -C=C-H(g) - NH 2 (am) .g -C=C - (am) + NH 3 (g) Sodium alkynides are extremely useful for the synthesis of other alkynes because the alkynide ion is a powerful nucleophile in reaction with haloalkanes (section 21.7.2.). (ii) Also, alkynes with terminal hydrogen atoms form silver and copper(I) salts when treated with diammine complex ions of the metals. The formation of characteristic precipitates makes the reactions useful tests for 1-alkynes: RC=CH(g) + Cu(NH 3 ) 2 + (aq) + - OH(aq) .g . RC=CCu(s) + H 2 O(l) + NH 3 (aq) red ppt. RC=CH(g) + Ag(NH 3 ) 2 + (aq) + - OH(aq) .g . RC=CAg(s) + H 2 O(l) + NH 3 (aq) white ppt. 21.4.5. Other reactions i) Oxidation: Like alkanes and alkenes, alkynes undergo various oxidation reactions, not least autoxidation and combustion. E.g. Combustion: 2CH=CH(g) . + . 5O 2 (g) .g . 4CO 2 (g) . + . 2H 2 O(g) 21.5. INDUCTIVE AND MESOMERIC EFFECTS 21.5.1. Introduction: In section 21.4.1. a new concept was slipped into the text without explanation. What is a negative inductive effect? For that matter, what is a positive inductive effect? Briefly, inductive effects, positive or negative, are little more than polarised bonds seen with a different journalistic bent. It is important to realise that even scientific language depends on the attitude of the observer. Inductive effects exist in s-bonds and also in p-bonds, but in the former case they do not involve delocalisation. Polarisations which do involve delocalisation via p bonding systems and p-orbitals are known as mesomeric or conjugative effects. In fact, mesomeric and conjugative effects are little more than delocalisation seen with a different journalistic bent. They do not even involve polarisation in all circumstances. Two further points on language: First, the different jounalistic bent described above is not totally artificial. It is useful for describing particular situations because it saves clumsy explanations. Good scientists would not make good Sun reporters, though they might do well on The Independent. Second, inductive and mesomeric effects are often talked about as "occurring". This does not mean that they occur on any time scale. The negative inductive effect of a halogen atom does not suddenly happen in a haloalkane; it is there all the time. 21.5.2. Inductive effects exist where (occur where) two atoms or groups which differ in electronegativity are bonded. A more electronegative atom or group exerts a negative inductive (-I) effect, "pulling" electrons towards itself and acquiring partial negative charge (d-). For example, chlorine and oxygen exert -I effects in the molecules shown here: The most important groups to exert an electron pushing, or +I, effect are alkyl groups. This is largely a characteristic of the large number electropositive hydrogen atoms within alkyl groups. 21.5.3. Mesomeric or conjugative effects exist: i) where p-bonding systems would otherwise be next to each other - separated by one single bond, or ii) where electrons in p-orbitals would otherwise be next to p-bonding systems - separated by one single bond. The term conjugation is often reserved for situations where the polarity of the effect is not relevant, eg in buta-1,3-diene (section 4.8.8.) The double bonds which appear in the simple bonding diagram (FIG. 4.13.) are conjugated and there is no polarisation. However, in phenylethene, it is more relevant to think of the ethene group exerting a positive mesomeric (+M) effect on the benzene ring. The p-bonding systems are conjugated, but in this case there is polarisation: Another way of describing the situation is to say that electrons from the alkene double bond are delocalised into the benzene ring. In phenylethanal, the carbonyl group is considered as exerting a negative mesomeric (-M) effect on the benzene ring. Electrons are delocalised out of the ring onto the electronegative oxygen atom: Thus doubly bonded electronegative elements exert negative mesomeric effects as well as negative inductive effects. In other cases, the polarity of mesomeric effects is determined by the relative electron densities of the p-bonding systems which overlap i.e. electrons delocalise from regions of higher electron density to regions of lower electron density, as in the case of phenylethene described above. 21.5.4. Positive or negative? In the last section we described a positive mesomeric effect in phenylethene; the ethene group exerts a positive mesomeric effect on the benzene ring. However, it would appear just as valid to say that the benzene ring exerts a negative mesomeric effect on the carbonyl group. Which is correct? Sometimes, either can be correct. Take phenylethene: If you are considering the reactions of the ethene group, it is probably most useful to think in terms of the benzene ring exerting a -M effect on the ethene group. If you are considering the reactions of the benzene ring, it is probably most useful to consider how the +M effect of the ethene group affects the benzene reactions. Alternatively, using the other form of language, "electrons from the double bond are delocalised into the benzene ring", covers both situations. In other cases when using the mesomeric terminology, one description certainly is better than the other. Thus in phenylethanal it would be artificial to describe the benzene ring exerting a positive mesomeric effect and "pushing" electrons onto the electronegative oxygen atom. Similar arguments apply to inductive effects. 21.6. BENZENE AND OTHER AROMATIC HYDROCARBONS 21.6.1. Predictions. The high electron density of the benzene ring's p-bonding system makes it susceptible to attack by electrophiles. However, despite being unsaturated, benzene does not undergo addition as a result of such attack. Addition would involve a concentration of the electron density within the benzene ring by breaking the aromatic delocalisation: Thus substsitution of a hydrogen by the attacking group is favoured, since this restores delocalisation. The overall reaction is therefore elctrophilic substitution via mechanism 5 (FIG. 20.1.). 21.6.2. Electrophilic substitution in the benzene ring: some examples. Table 21.3. Examples of electrophiles which react with benzene. Electrophile reagent conditions main organic products NO 2 + conc. nitric dissolved in conc. sulphuric 55°C g 100°C g reflux 48hrs g nitrobenzene 1,3-dinitrobenzene 1,3,5-trinitrobenzene *SO 3 conc. sulphuric acid 80°C Benzenesulphonic acid d+ . d- AlCl 3 , FeBr 3 etc, Cl-Cl--AlCl 3 Br-Br--FeBr 3 R-Hal--AlCl 3 chlorine g bromine g haloalkane g catalyst (or iron filings g FeBr 3 ) (Lewis acids) g chlorobenzene g bromobenzene g alkylbenzene, but difficult to stop at mono substituted stage * RCO-Cl--AlCl 3 acid chloride Lewis acid halogen carrier, as above. phenylketone (mono!) can be reduced to corresp. alkyly deriv. eg by Zn amalgam/HCl. + CH 3 =CH 2 ethene acid to protonate alkene, (HCl/H 3 PO 4 ), plus Lewis acid ethylbenzene (mono!) Industrially: +Zn/600°C g phenylethene (styrene) * marks the electron deficient centre in the electrophile (if not already obvious). 21.6.3. Effects of the rest of the molecule. The table shows that it is easy to stop some reactions at the mono substitution stage, but difficult to stop others. This highlights an interesting piece of theory. Electron pushing and electron withdrawing groups attached to the benzene ring affect its reaction with electrophiles in two ways: i) they make it either more reactive (activate) by increasing electron density in the ring, or less reactive (deactivate) by decreasing the electron density in the ring; ii) they direct electrophiles to particular positions in the ring by changing the distribution of electron density i.e. by making it more concentrated around particular carbon atoms. Electrophiles are more likely to attack in these positions. Looking at this in more detail emphasises a point about models. It is easiest to see how these effects come about by using a model of the benzene ring which looks less like the real thing than the model which shows delocalisation. Models are only models and, as previously stated, different models serve different functions. 21.6.4. Positive mesomeric effects e.g. NH 2 group. i) Electron pushing groups activate the ring towards electrophilic attack because the electron density is increased. ii) They are also 2,4,6-directing because the electron density is increased more in the 2,4 and 6 positions. Other groups which similarly activate and 2,4,6-direct are: -OH, -OR, -NHR, -NR 2 , -C 6 H 5 , etc. 21.6.5. Negative mesomeric effects e.g. -COR. i) Electron withdrawing groups deactivate the ring towards electrophilic attack because the electron density is decreased. ii) They are also 3,5-directing, because the electron density is reduced less in the 3 and 5 positions positions. Other groups which similarly deactivate and 3,5-direct are: -COOH, -NO 2 , -C=N, -SO 3 H, etc. 21.6.6. Inductive and combined effects The same effects can be brought about by inductive mechanisms. The most important groups to exert a positive inductive effect on the ring are alkyl groups: Groups which exert a negative inductive effect are -CF 3 , -CCl 3 etc. Obviously, +I effects activate and 2,4,6-direct, and -I effects deactivate and 3,5-direct. However, some groups exert both mesomeric and inductive effects. In the case of the nitro group this is simple: it exerts both a -M and a -I effect. However, the halogens, for example, exert a +M effect, but a -I effect. As a result, halogen atoms 2,4,6- direct, but weakly deactivate the benzene ring.