Orientation, of addition

We will use the addition of hydrogen bromide to ethene as an example of the addition of an unsymmetrical molecule to a symmetrical substrate. 

Write down the overall equation for this addition reaction. 

Indicate the direction and nature of the dipole in the hydrogen bromide molecule, and so suggest which end of this molecule is attacked by the electron rich n orbital of the ethene. Then draw a reaction mechanism that illustrates the movement of electrons that results in the formation of the charged intermediate. 

The bromine atom is more electronegative than the hydrogen atom, and so the hydrogen carries a 5+ charge. This means that it is the hydrogen that is attacked by the n electrons to form the carbonium ion. As a result, a bromide ion is formed. Furthermore, the lowest energy form of this carbonium ion 

Suggest how this reaction pathway might continue, and draw the structure of the product. 

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In the case of the reaction between 2-diazopropane and diphenyldiacetylene, the reverse (as compared with other diynes) orientation of addition of the first molecule of the diazo compound with a predominant formation of 4-phenylethynylpyrazole is observed. Therefore, it is noteworthy that whereas the regioselectivity of the addition of diazoalkanes to alkenes is well studied audits products have, as a rule, the structure been predicted with respect to electron effects, the problem of orientation... 

Look carefully at the reactions shown in the previous section. In each case, an unsymmetrically substituted alkene has given a single addition product, rather than the mixture that might have been expected. As another example, 1-pentene might react with HC1 to give both 1-chloropentane and 2-chloropentane, but it doesn t. Instead, the reaction gives only 2-chloropentane as the sole product. We say that such reactions are regiospecific (ree-jee-oh-specific) when only one of two possible orientations of addition occurs. 

It might seem that allylic functionalisation can be used only on terminal alkenes such as (26) or trisubstituted alkenes, such as (31) when the orientation of addition is unambiguous. 

A very similar situation is encountered in the electrophilic addition of unsymmetrical adducts (e.g. HBr) to vinyl halides (e.g. CH2=CHBr), where the inductive effect of halogen controls the rate, but relative mesomeric stabilisation of the carbocationic intermediate controls the orientation, of addition (p. 185). 

A bridged intermediate exactly analogous to a bromonium ion cannot be formed as H has no electron pair available, but it may be that in some cases a n complex (21) is the intermediate. We shall, however, normally write the intermediate as a carbocation, and it is the relative stability of possible, alternative, carbocations (e.g. 23 and 24) that determines the overall orientation of addition, e.g. in the addition of HBr to propene (22) under polar conditions ... 

Electrophilic addition to 1-haloalkenes (e.g. 27), presents a number of parallels to the electrophilic substitution of halobenzenes (p. 155). Thus it is the involvement of the electron pairs on Br that controls the orientation of addition (cf. o-/p-direction in C6H5Br) ... 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

The formation of the carbocationic intermediate (37), either directly or via an initial it complex, appears to be rate-limiting, and the overall orientation of addition is Markownikov. There is evidence of some ANTI stereoselectivity, but this is not very marked and is dependent on the alkene and on the reaction conditions. 

With unsymmetrical dienes (74a and 74b) and unsymmetrical adducts, the problem of orientation of addition (cf. p. 184) arises. Initial attack will still be on a terminal carbon atom of the conjugated system so that a delocalised allylic intermediate is obtained, but preferential attack will be on the terminal carbon that will yield the more stable of the two possible cations i.e. (75) rather than (76), and (77) rather than (78) ... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

While at Leeds from 1924 to 1930, Ingold s laboratory focused on three main topics of research (1) the nature and mechanism of orienting effects of groups in aromatic substitution (mainly nitration) (2) the study of prototropic rearrangements (shifts of H+) and aniontropic rearrangements (shifts of anions) as the ionic mechanisms of tautomerism and (3) the effect of polar substituents on the velocity and orientation of addition reactions to unsaturated systems. One of Ingold s students at Leeds, John William Baker, wrote a widely read book on tautomerism. 16... 

EFFECT OF SUBSTITUENTS ON RATE OF ADDITION, p. 182. ORIENTATION OF ADDITION, p. 184. 

The vinyl halide product is then able to react with a further mole of HX, and the halide atom already present influences the orientation of addition in this step. The second halide adds to the carbon that already carries a halide. In the case of the second addition of HX to RC CH, we can see that we are now considering the relative stabilities of tertiary and primary carbocations. The halide s inductive effect actually destabilizes the tertiary carbocation. Nevertheless, this is outweighed by a favourable stabilization from the halide by overlap of lone pair electrons, helping to disperse the positive charge. 

Note also that, if 2 mol of HX add to an alkyne, it is of no consequence whether the first addition produces an alkene with E or Z stereochemistry, since the orientation of addition means the final product has no potential chiral centres. 

Nucleophiles react particularly easily with quaternized azines and with pyrylium and thiopyrylium slats (cf. equation 23) typical examples, including the well-known reaction of pyridinium salts with hydroxide in the presence of potassium ferricyanide to give 2-pyridones, are summarized in equations (34)-(36) (note the rather unusual orientation of addition in the last case reaction normally occurs essentially exclusively a to the heteroatom if the position is free or occupied by a leaving group). 

Enol ethers and enol acetates of cyclic 1,3-dicarbonyl compounds also afford mixtures of regioisomers on irradiation in the presence of allene, the preferential orientation of addition for enol ethers being mainly head-to-tail.9-11 Endocyclic enol ethers on the other hand, e.g. 2.3-dihydro-4//-pyran-4-ones 4, add regioselectively to allene with exclusive formation of head-to-head adducts.11... 

Electronic and steric factors seem to determine both the site and the orientation of addition of diazoalkanes to conjugated enynes. Although rates of addition of DPD to ethene and ethyne bearing identical single substituents are approximately the same,48 addition to butenyne occurs almost exclusively at the double bond.46... 

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