Living on Acetylene. A Primordial Energy Source - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 17

acetylene can be used to quantify denitrification rates [ 2 ]. Acetylene itself is a
highly flammable gas that forms explosive mixtures with air over a wide range of
concentrations (2.4-83 % vol, material safety datasheet, Air Liquide GmbH,
Germany). Set under pressure, it can polymerize spontaneously in an exothermic
reaction [ 2 ].

2.1 Properties of Acetylene

The physical and chemical properties of acetylene are mainly determined by the
carbon-carbon triple bond. Formed by the overlapping of one of the sp hybrid
orbitals and two p orbitals of each of the sp hybridized carbon atoms the bond
consists of one

C bond angle is
180 C with a bond length of 121 pm for the triple bond. The triple bond increases
the electronegativity of the carbon atoms and thereby the acidity of the H-C bonds
of acetylene compared to ethylene and ethane, resulting in a p K a of 24 for acetylene
compared to 44 for ethylene [ 2 ]. This acidity is manifested in the formation of
heavy metal acetylides (e.g., Cu(I) acetylide or Ag(I) acetylide) by reaction of
acetylene with the corresponding metal cations. In addition, alkali metal acetylides
(e.g., sodium acetylide) are formed by reaction of the elemental metal with
acetylene. Acetylene can also serve as a ligand to transition metals. Both
σ

σ -bond and two orthogonal ˀ -bonds. The H-C

-bonded and ˀ -bonded systems have been described. Examples include

(HC -C 5 H 5 -Ni] 2 C 2 H 2 [ 2 ].

In general, the chemistry of acetylene is rather rich and diverse. Reactions
of acetylene include reduction and oxidation as well as electrophilic and nucleophilic
additions [ 4 ]. Due to the electron configuration of the C

CH) 2 NiBr 2 ,HC CH-Ni(CN) 2 and [ ˀ

C triple bond, electrophilic
additions to alkynes are much slower compared to additions to alkenes. On the other
hand, nucleophilic additions are much faster than those on alkenes [ 5 ]. The reason for
this is that the electrons of the carbon atoms are massed in the triple bond, leaving the
“backside” of the cores of the carbon atoms open for nucleophiles to attack.

2.2 Sources and Bioavailability of Acetylene
on Earth and Other Planets

Today, acetylene is only a minor trace gas in the composition of the Earth's
atmosphere. Depending on where the samples were taken, concentrations of
acetylene between 0.02-0.08 ppbv were detected [ 6 ]. Since acetylene on Earth
seems to be mainly of anthropogenic origin, with exhaust from combustion engines
as main source, samples from oceanic and rural areas show less abundance of
acetylene [ 7 ].