Alkynes, acid strength

A graphical representation of the dependence of the activity per acidic site of the zeolites on their Si/Al ratio is given in Fig. 2. It is known that zeolite acidity is enhanced by incrasing the Si/Al ratio (ref. 17,18). Therefore, the obtained linear correlation strongly suggests that the activity of zeolites in hydration of alkynes is a direct and single function of the acid strength and is not dependent of the zeolite framework. Such a result is consistent with the correlation obtained in olefin hydration (ref. 19). 

Oxymercuration reactions are usually carried out in a three-component system involving an alkene, alkyne, or cyclopropane, a mercury compound HgX2, and a nucleophile HY, where HY may be HOH, ROH, HOOH, HOOR, or HOCOR and is frequently the solvent . Reactivity follows the sequence of Lewis acid strength in HgX2, with Hg(OCOMe)2 < Hg(OCOCF3)2, Hg(C104)2, and a steric sequence in alkene, with CH2=CH2 > RCH=CH2 > cis-RCH=CHR > trans-RCH=CHR > R2C=CHR > R2C=CR2. 

In analogy to alkynes, alkenes, and alkanes, the three compound types differ in the hybridization of the nitrogen atom. Acid strengths vary in the same way ... 

Pairing an alkyne with a more conventional proton acceptor does appear to form true H-bonds in certain cases. Using the red shift of the C—H stretch as a barometer of H-bond strength, there is a continuum that depends upon the strengths of the acid and the base. 

One of the many important differences between phosphorus and nitrogen chemistry is the relative strengths of their bonds to hydrogen. The relatively weak P—H bond means that this functionality can be added across a wide variety of unsaturated molecules (alkenes, alkynes, carbonyls) and hence this represents an excellent method for preparing tertiary phosphines. The addition of P 11 compounds to C=0 and C=N has been described in detail by Gilheany and Mitchell.2 The reaction can be catalyzed by base (potassium hydroxide, butyllithium), acid (HC1, carboxylic acids, sulfonic acids, boron trifluoride), free radical (uv, organic peroxides, AIBN) or metal (simple metal salts, late transition-metal complexes). In some circumstances no catalyst is required at all for P 11 additions to proceed.60... 

If one 7i-bond of an alkyne reacts as a base, it is quite reasonable to ask if it is a stronger or weaker base relative to the ji-bond of an alkene. An alkyne is a weaker base than an alkene. It is known that one Ji-bond of a carbon-carbon triple bond is about 8.8 kcal mol i (36.8 kJ moH) weaker than the Ji-bond in an alkene. In other words, the alkyne Ji-bond has less electron density and it is less able to donate electrons to an acid (i.e., it is a weaker base). Some differences in chemical reactivity between alkynes and alkenes can be predicted by understanding this difference in base strength. 

Another important factor for a successful formation of esters is the strength of the acid used. Thus, the reaction of alkynes with carbon monoxide and water /HOR failed nearly completely with acetic acid (P 4.7) or pivalic acid (P 5). The reaction products obtained are exclusively the unsaturated acids. Even the strength of chloroacetic acid (P 2.8) is unsufficient equal amounts of acids and esters are formed. However, with hydrochloric acid (Pk> 0) high yields of unsaturated esters are obtained [367]. 

The final paragraph of a paper on the preparative and structural chemistry of amino-acid-alkene mixed complexes of platinum(n) reports some qualitative observations on racemization rates of complexes of formula [Pt(aIkene)(L-prolinate)a]. These rates correlate with platinum-olefin bond strengths. MO calculations of barriers to internal rotation in platinum-alkene and -alkyne complexes have been made. ... 

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