Acetylenic bonds

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

Ethylene has a carbon-carbon double bond acetylene has a carbon-carbon triple bond... 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Acetylene.. H—C= C—H, the simplest alkyne, was once widely used in industry as the starting material for the preparation of acetaldehyde, acetic acid, vinyl chloride, and other high-volume chemicals, but more efficient routes to these substances using ethylene as starting material are now available. Acetylene is still used in the preparation of acrylic polymers but is probably best known as the gas burned in high-temperature oxy-acetylene welding torches. 

Ethane, a component of natural gas, contains a C—C single bond. Ethylene, widely used to make plastics, contains a CDC double bond. Acetylene, used as fuel for welding torches, contains a CDC triple bond. 

Logically, more bonds between atoms should mean a greater force of attraction and increased carbon density. But doubly bonded alkenes are more reactive, not less reactive, than singly bonded alkanes or triply bonded acetylenic compounds. Further, how could the force associated with a polyvalent atom be divided a priori to react to the atoms encountered, sometimes dividing itself in half, other times by thirds To assume that valences act across empty space, free of atoms, some chemists noted, "is only possible on paper or in a model where there are lines or wires but not forces. "89... 

The presence of the endothermic triply-bonded acetylene (ethyne) group confers explosive instability on a wide range of acetylenic compounds (notably when halogen is also present) and derivatives of metals (and especially of heavy metals) [1], Explosive properties of butadiyne, buten-3-yne, hexatriyne, propyne and propadiene have been reviewed, with 74 references [2], The tendency of higher acetylenes to explosive decomposition may be reduced by dilution with methanol [3], The class includes the separately treated groups ... 

Aliphatic- Straight- or branch-chain organic molecules that have saturated bonds (paraffins), double bonds (olefins), or triple bonds (acetylenes). 

There are also carbon-carbon triple bonds. Acetylene (or, more rigorously, ethyne—yne indicating the presence of a triple bond in the molecule), C2H2, is a common hydrocarbon that contains a triple bond HC=CH. Note that here too each carbon atom makes four chemical bonds the triple bond counts as three. Acetylene finds significant commercial use in oxyacetylene torches. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Section 1.4 Many organic compounds have double or triple bonds to carbon. Four electrons are involved in a double bond six in a triple bond. H H =c H—c=c—H H H Ethylene has a carbon-carbon double bond acetylene has a carbon-carbon triple bond. 

The presence of the endothermic triply-bonded acetylene (ethyne) group confers explosive instability on a wide range of acetylenic compounds (notably when halogen... 

Both the lithium sulfur dioxide (Li-SO and lithium thionyl chloride (Li-SOCy cells may be classified as liquid cathode systems. In these systems, S02 and SOCl2 function as solvents for the electrolyte, and as the active materials at the cathode to provide voltage and ampere capacity. As liquids, these solvents permeate the porous carbon cathode material. Lithium metal serves as the anode, and a polymer-bonded porous carbon is the cathode current collector in both systems. Both cells use a Teflon-bonded acetylene black cathode structure with metallic lithium as the anode. The Li-S02 is used in a spirally wound, jelly-roll construction to increase the surface area and improve... 

Alkynes are hydrocarbons in which there is at least one triple bond. Acetylene is an alkyne that you have likely heard of, as in an acetylene torch. Its formula is... 

M—has also been reported for olefins and acetylenes ir-bonded to rhodium and to platinum (6, 21, 46, 87). In the case of rhodium, iy(i°3Rh—is between 10 and 16 Hz for a 7r-bonded olefin (see Table XXVII), while for the cr-bonded carbon in [(C5H5)Rh(ff-C3Hs)-(w-CsHb)], 7( ° Rh—is 26 Hz. It was suggested the bonding of the olefin results from a 60% contribution from a dsp -vnet X orbital and sp -carbon orbital 21). For the olefins and acetylenes w-bonded to platinum 7( Pt—is between 18 and 195 Hz (see Table XXIX) compared to the range of 360 to 1000 Hz reported for carbon cr-bonded to platinum. It was found that 7( Pt— C) is less for a 7r-bonded acetylene than for a rr-bonded ethylene. This was considered as evidence for the Chatt-Dewar-Duncanson molecular orbital model 39, 63) of TT-bonding (XIV), rather than the formally equivalent valence-bond treatment, (XV) and (XVI) 46). However, no allowance appears to have been made for the effect on the hybridization at the carbon of the pseudo-... 

Scheme 5 is a summary of the experimental frequencies of the v(CH), v(C = Q, and v(OH) modes for the 1 1 7i-bonded acetylene and methylacetylene complexes. Notice that the shift of v(C = C) and v(OH) bands is always negative with respect to the free hydrocarbon molecules (v(C = C) = 1974 and 2142cm for acetylene and methylacetylene, respectively) or Bronsted acid group, respectively. The shift of the v(OH) is a normal consequence of hydrogen-bonding and has been abundantly documented for many bases adsorbed molecularly in zeolites with proton affinities in the range of 420—840kJmol (20,21,156-161). Furthermore, the v(C = C)... 

The C-H and C-C cr-bonds are all trigonal sp hybrids, with 120° bond angles. The two unhybridized p-orbitals form a 7r-bond, which gives the molecule its rigid planar structure. The two carbon atoms are connected by a double bond, consisting of one o and one tt. The third canonical form of 5/ -hybridization occurs in C-C triple bonds, for example, acetylene (ethyne). Here, two of the p AO s on each carbon remain unhybridized and can form two n -bonds, in addition to two (r-bonds. Acetylene H-C=C-H is a linear molecule, as shown below, since the p-hybrids are oriented 180° apart. 

Although acetylene adds four monovalent atoms while ethylene adds only two, it does not follow that the greater unsaturation is accompanied by a more rapid rate of addition. As a rule, the activity of the triple bond, measured by the rate of addition, is lower than the activity of the double bond. Acetylene forms explosive mixtures with air, and care should be exercised by the student to keep the generator away from all flames. 

Simple hydrogen-bonded acetylenes such as propioltunide (5) and propiolaldehyde semicarbazone (6), whose crystal structures reveal short contacts between acetylenic ctubons, are unreactive to gamma radiation when the crystals are grown by vacuum sublimation (21). Polymerization of these compounds proceeds to a low degree of conversion for crystals grovm in chlorinated solvents. 

Acetylene is more strongly adsorbed than ethylene, and this must be accommodated in the proposed structure for the adsorbed state. However, both Structures (I) and (II) fulfill this requirement. Di-a-bonded acetylene is expected to be more strongly bound to the surface than di-CT-bonded ethylene because the first 7r-bond in acetylene is weaker than the second (2, 74). For the 7r-adsorbed structures, acetylene, can bond with two metal atoms whereas ethylene may only bond with one, so again acetylene is reasonably expected to be the more strongly adsorbed species. 

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