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Can Life exist on Silicon instead of Carbon?

February 27th, 2017

Since the time around 1900, some scientists considered as to whether living creatures could use other elements as building blocks instead of carbon. As alternatives, life forms based on silicon, boron, nitrogen or sulfur have been proposed, which under certain conditions can also form stable macromolecules. In particular, silicon-based life is discussed seriously and is also very popular in science fiction. Among the possible alternatives, silicon is most similar to carbon. But how does it look like from the chemical point of view? Can life based on silicon actually exist?

Figure 1: Life can take a variety of forms, but can biochemistry also work on the basis of silicon? At least as a building material, silicon is already used by terrestrial organisms, e.g. of diatoms. This image shows fractals that resemble diatom skeletons, in order to have a nice picture at the top of an entry about chemistry ;) .

Two things are particularly important for a chemistry of life. The "skeleton" or backbone, whether made of carbon or silicon, must be stable in a certain range of environmental conditions and should not dissolve immediately upon contact with water or any other natural solvent. At the same time, the bonds must not be too strong, so that energy can be transferred or new bonds can be formed with relative ease. All life processes are based on the formation and dissolution of various types of interatomic and intermolecular bonds. If the bonds become too strong, then life comes to an end. If the bonds dissolve too easily, everything falls apart and the organism dissolves or burns.

In addition to the skeletal backbone, the greatest possible number of functional groups is required. A pure hydrocarbon backbone is almost inert to chemical reactions (except burning). Only functional groups enable a controlled biochemistry. Frequently occurring functional groups are e.g. alcohols, ketones, phosphates, amines, ethers, esters and many others. The properties of the functional groups are often more important than the details of the backbone. For example, the hydrocarbons methane, ethane and propane are gaseous at room temperature. However, the addition of the functional alcohol group (-OH) to these backbones generates liquids which can be mixed with water and which have a chemistry different from that of pure hydrocarbons.

In order to compare carbon and silicon in this respect, we must consider three important points: the atomic orbitals, electronegativity and the strength of the bond to other important elements.
As regarding atomic orbitals, silicon is very similar to carbon. In their outer shell, both elements have two electrons in the s orbital and two electrons in the p orbitals. To achieve the stable noble gas configuration (consisting of 8 electrons filling both the s and p orbitals), both elements can either give away four electrons or absorb four electrons. Altogether this results in four possible bonds, which are arranged in space like the corners of a three-sided pyramid (this form is called a tetrahedron). In addition, silicon can also provide other atoms with the d-orbitals underneath for binding. These orbitals are not occupied in elementary silicon, but are nonetheless available as empty orbitals for bindings. This explains the 5- or 6-bond of some silicon compounds. For organic chemistry, which requires the greatest possible diversity of structures, this would not necessarily be a hindrance.

Electonegativity is a quantity used to describe the attraction of atomic nuclei to electrons within molecules. Of all chemical elements, fluorine has the highest electronegativity (3.98 on the Pauling scale), which means that fluorine in a compound attracts the electrons most strongly. The alkali metals on the other hand (the metals from lithium to radium), have electronegativity values between 0.7 and 0.98, and thus have the least tendency to bind electrons. On the contrary, the alkali metals give off their only outer s-electron very readily, which is why they are very reactive. Carbon and silicon exhibit both medium-strong electronegativities: carbon 2.55 and silicon 1.9.

Figure 2: Comparison of methane and silane as basic units of a possible biochemistry. The molecules are shown here flat, the actual structure corresponds to a tetrahedron. The numbers on the atoms represent the respective electronegativity values, d + / d- denotes the distribution of polarized charge. The arrow indicates the shift of electron density. The silicon-hydrogen bond is polarized exactly opposite to the carbon-hydrogen bond. Together with the weaker bond, this makes the silane molecule unstable in the presence of oxygen or water.
An interesting difference is already apparent when one considers the simplest connection of carbon and silicon with hydrogen. Methane (CH4), a gas at room temperature, is the simplest hydrocarbon and as such the basic constituent of higher hydrocarbons and biochemically interesting compounds. The corresponding compound of silicon and hydrogen is the gaseous silane (SiH4).

The electronegativity of hydrogen (2.2) is exactly between that of carbon and silicon. While the carbon atom in the methane molecule pulls the electrons towards it and leaves the four hydrogen atoms with a net electron deficit and a slightly positive partial charge, it is exactly reversed in silane. There, the four hydrogen molecules attract the electrons stronger than does the silicon atom, which gives them a slightly negative charge and the silicon atom a slight electron deficiency and positive partial charge.

Chemically, this is very significant. In a mixture with air, both gases are surrounded by inert nitrogen and reactive and also very electronegative oxygen (electronegativity: 3.44). Methane burns with oxygen to water and carbon dioxide, but requires an activation energy, e.g. in the form of a spark. Without such an ignition, the oxygen molecules mainly come into contact with the electron-deficient hydrogen atoms of the methane molecule - there is "nothing to fetch" for oxygen, the mixture remains stable. It is only with the aid of a spark that oxygen can break the bond between carbon and hydrogen and react with both.

It is exactly the other way round with a mixture of oxygen and silane. Here, oxygen is in contact with electron-rich hydrogen atoms. Together with the somewhat weaker silicon-hydrogen bond, this is sufficient to start the reaction by itself: oxygen breaks the bond between silicon and hydrogen, reacts with hydrogen to form water and with silicon to form silicon dioxide (SiO2, glass). Silane is pyrophoric, which means it burns on its own when in contact with oxygen.
Within longer-chain molecules, the chemical bond between two silicon atoms is about one quarter weaker than the bond between two carbon atoms. This is a consequence of the larger radius of the silicon atom: the bonding electrons are farther away from the positive charge of the atomic nucleus, thus the attracting forces are weaker. At the same time, the bond between silicon and oxygen is extremely strong, the two elements are very difficult to separate from each other.

Longer-chain hydrocarbons are either liquids or solids at room temperature. Hexane, a molecule containing 6 carbon atoms, is one possible form, candle wax (paraffin) another. Hexane is a liquid lighter than water and floats on top of it but does not react with water. Also, a piece of candle wax will not change when put in water. For longer-chain silanes, however, this is only true in the absence of oxygen. Without oxygen, silanes can be stably stored on saline, aqueous solutions. If, however, oxygen is added, this results in the decomposition of the silanes. In alkaline solutions, the reaction proceeds even faster with formation of hydrogen and silicon hydroxide (Si(OH)4).

We might as well compare the ability to form double and triple bonds and find that carbon has much more potential for that than silicon. Both types of binding are of great importance for biochemistry because they allow for additional structural and chemical diversity. The topic of structural diversity in potential silicon-based life is covered in more detail in Dirk Schulze-Makuch's and Louis Irwin's book "Life in the Universe. Expectations and Constraints."

However, at this point, we might as well make a break and consider the likelihood of the natural development of a silicon biochemistry compared to a carbon biochemistry. Carbon forms thousands of compounds which remain stable in the presence of oxygen or in aqueous solution long enough to allow for a functioning biochemistry; many carbon compounds have already been detected in interstellar space. Burning or oxidation results in gaseous carbon dioxide, which is soluble in water and is thus accessible to further chemical reactions. However, the analogous compounds of silicon are unstable in the presence of oxygen and water. The product of oxidation is glass, which hardly reacts with anything at all.

Within the gas nebulae from which stars and planets are formed, oxygen is the third most common element. Within the Earth's crust it is even the second most common. Water is also found everywhere in the universe and has already been detected in quasars, protostellar nebulae and exoplanets. Even on a planet without free oxygen and without water, oxygen would still be bound within the structure of minerals. The most important components of the Earth's mantle are forms of silicon oxide, magnesium oxide and aluminum oxide. The probability that any precursors of a silicon biochemistry on other planets would end up as glass is thus very high and effectively kills the idea of silicon life. Such life would thus remain in the realm of science fiction. Carbon is a much more suitable element for this function.
Clayden, J., Greeves, N., & Warren, S. G. (2013). Organische Chemie. Springer Spektrum.
Schulze-Makuch, D., & Irwin, L. N. (2008). Life in the universe: expectations and constraints. Springer Science & Business Media; pp 81ff

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