Propyne hydrogen bonding

There are a few pyrimidine-modified DNA structures. 5-(2-Hydroxyethyl)-dU was substituted for thymidines in a quadruplex structure. The presence of the hydroxyethyl groups allows for additional hydrogen bonding within the expected tetrads. A study of the effect of introducing C5-propyne groups showed that the propyne groups stack on the aromatic ring of the... 

A line structure (introduced in Section C) represents a chain of carbon atoms as a zigzag line. The end of each short line in the zigzag represents a carbon atom. Because carbon nearly always has a valence of 4 in organic compounds, we do not need to show the C—H bonds. We just fill in the correct number of hydrogen atoms mentally, as we see for methylbutane (6), isoprene (7), and propyne (8). As explained in Section 2.7, a benzene ring is represented by a circle inside a hexagon, and we need to remember that one hydrogen atom is attached to each carbon atom. 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

The pyrolysis of pyrrole produces a variety of products hydrogen cyanide, propyne, allene, acetylene, c/ -crotonitrile, and allyl cyanide, among them. Lifshitz et al. hypothesized that pyrrole undergoes 1,2-bond (N—C) cleavage, then an internal H-atom transfer, to yield a radical intermediate that can isomerize to either c/ -crotonitrile or allyl cyanide, or dissociate to HCN and propyne.Bacskay et al. completed quantum chemical comparisons of the isoelectronic pyrrolyl and cyclopentadienyl radicals they hypothesized that pyrrolyl radical is formed via C—H bond scission in the intermediate pyrrolenine (2/f-pyrrole) rather than directly via N—H bond cleavage (Fig. 14). Mackie et al. explained a similar finding, postulating that it was the formation of pyrrolenine that dictated the rate at which pyrrole pyrolysis occurred. 

Acetylene is the most widely used alkyne. Alkynes undergo the same reactions as alkenes. Because a triple bond coimects the two carbon atoms, addition of an atom initially forms a double bond. The addition of a second atom converts the double bond into a single bond. For example, hydrogenation of propyne to propane is represented as ... 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

The proper double-bond stereochemistry may be achieved by using 2-heptyne as a reactant in the final step. Lithium-ammonia reduction of 2-heptyne gives the trans alkene hydrogenation over Lindlar palladium gives the cis isomer. The first task is therefore the alkylation of propyne to 2-heptyne. 

The chemical properties and uses of propargyl alcohol has three potentially reactive sites (1) a primary hydroxyl group (i.e., CH2OH), (2) a triple bond (-C=C-), and (3) an acetylenic hydrogen (-C=CH) that makes the alcohol an extremely versatile chemical intermediate. The hydroxyl group can be esterified with acid chlorides, anhydrides, or carboxylic acids, and it reacts with aldehydes or vinyl ethers in the presence of an acid catalyst to form acetals. At low temperatures, oxidation with chromic acid gives propynal or propynoic acid ... 

The mechanism of the Au(III) catalysis proposed in Scheme 5 implies the stereoselective formation of the new C-C bond which, of course, cannot be observed in the final product when terminal alkynes are used (the aryl group and the former alkyne hydrogen are situated at the same side of the double bond in the vinyl-Au intermediate). For the reaction of 1-phenyl-l-propyne and mesitylene 1 (see below, Table 1) the proposed mechanism should lead to preferential formation of the Z isomer which is, in fact, observed [2]. The formation of a small amount of E isomers can be explained by isomerization of the initially formed Z compound. Such isomerization was, in fact, observed directly in the case of related electron-poor alkynes [4],... 

Both the carbon atom and the nitrogen atom of the cyano group are sp hybridized, and the R—C = N bond angle is 180° (linear). The structure of a nitrile is similar to that of a terminal alkyne, except that the nitrogen atom of the nitrile has a lone pair of electrons in place of the acetylenic hydrogen of the alkyne. Figure 21-1 compares the structures of acetonitrile and propyne. 

The three carbon atoms of propyne lie in a straight line the bond angle is 180°. The H-C1HC2 bond angle is also 180°. The bond angle between hydrogen and the p3-hybridized carbon is 109°. 

The formation of propyne and allene by pyrolysis of cyclopropene arises from opposite [1,2]H shifts in diradicals 191 or 192 The substantially larger activation energy (by some 24.5 kJ mol" ) for formation of allene reflects differences in the transition state structures for the two processes. Thus, the propyne-forming reaction requires the migrating hydrogen atom to span a single bond (see 194), whilst in the allene-forming process a double bond is involved and a more strained situation ensues (see 195). The formation of but-2-yne from 3-methylcyclopropene is similarly rationalized but the... 

When propyne reacts with H2 under the proper conditions, the triple bond is broken and hydrogen atoms are added to the alkyne to form an alkane. 

Some other molecules containing triply bonded atoms are nitrogen, N=N , hydrogen cyanide, H—C=N , and propyne, CH3—C=C—H. In each case, both atoms involved in the triple bonds are sp hybridized. In the triple bond, each atom participates in one sigma and two pi bonds. The C atom in carbon dioxide, .0=C=0., must participate in two pi bonds (to two different O atoms). It also participates in two sigma bonds, so it is also sp hybridized and the molecule is linear. 

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

We have just seen that a conjugated diene is more stable than an isolated diene. Now we need to see why a cumulated diene is less stable than an isolated diene. Cumulated dienes are unlike other dienes in that the central carbon is sp hybridized since it has two tt bonds. In contrast, all the double-bonded carbons of isolated dienes and conjugated dienes are sp hybridized. The sp hybridization gives the cumulated dienes unique properties. For example, the —AH° for hydrogenation of allene is similar to the —AH° for the hydrogenation of propyne, a compound with two sp hybridized carbons. 

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