chemistry

Silenes

Silenes is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Silenes rather than just read about it. In short: In inorganic chemistry, silenes, or disilalkenes, are silicon compounds that contain Si=Si double bonds, where the oxidation state of Si is +2. The parent molecule is disilene, Si2H4.

Key takeaways

Silenes belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Silenes to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Silenes from memory before moving on to harder problems.

Reference excerpt

In inorganic chemistry, silenes, or disilalkenes, are silicon compounds that contain Si=Si double bonds, where the oxidation state of Si is +2. The parent molecule is disilene, Si2H4.

Structure The first transient disilene was reported in 1972 by D. N. Roark and Garry J. D. Peddle. Simple disilenes easily polymerize. To suppress this tendency, bulky substituents are used. Indeed the first isolable disilene, tetramesityldisilene, was described in 1981 by West, Fink, and Michl. It was prepared by UV-photolysis of the related cyclic trisilane:

2 [Si(mesityl)2]3 → 3 (mesityl)2Si=Si(mesityl)2

Structure of tetramesityldisilene Tetramesityldisilene (C6H2(CH3)3)2Si=Si(C6H2(CH3)3)2 is a yellow-orange solid. The Si=Si double bond lengths of disilenes vary between 2.14 and 2.29 Å and are nearly 5 to 10% shorter than the Si-Si single bond lengths of corresponding disilanes. A peculiarity of disilenes is the trans-bending of the substituents, which is never observed in alkenes. The trans-bent angles of disilenes between the R2Si planes and the Si=Si vector range from 0 to 33.8 °. This distortion is rationalized by the stability of the corresponding silylene fragments, although disilenes do not typically dissociate. The distorted geometry of disilenes can be rationalized by considering the valence orbitals of silicon, which are 3s and 3p, whereas those of carbon are 2s and 2p. Thus, the energy gap between the ns and np orbitals of a silicon atom is larger than that of a carbon atom. Therefore, silylene fragments are in a singlet state, while carbene fragments are in a triplet state. So, when double bonds are formed by the interaction of these two fragments, disilenes which consist of two silylene units are trans-bending and alkenes which consist of two carbene units are planar. The bending is even more extreme for the tin analogues of disilenes.

Synthesis Disilenes are generally synthesized by reduction of 1,2-dihalodisilane, by retro-Diels–Alder fragmentation, by dimerization of silylenes, by photofragmentation of cyclopolysilanes, or by rearrangement of silylsilylenes. A series of 1,1,1,4,4,4-hexaalkyl-2,3-bis(trialkylsilyl)tetrasil-2-enes structural analogues are typically synthesised using reductive coupling of the corresponding 1,1,1,3,3,3-hexaalkyl-2,3-tribromotrisilanes.

IUPAC names To form the root of the IUPAC names for silenes, simply change the -an- infix of the parent to -en-. For example, SiH3-SiH3 is the silane disilANe. The name of SiH2=SiH2 is therefore disilENe. In higher silenes, where isomers exist that differ in location of the double bond, the following numbering system is used:

Number the longest silicon chain that contains the double bond in the direction that gives the silicon atoms of the double bond the lowest possible numbers. Indicate the location of the double bond by the location of its first silicon. Name branched or substituted silenes in a manner similar to silanes. Number the silicon atoms, locate and name substituent groups, locate the double bond, and name the main chain.

See also Disilane Disilyne Organosilicon chemistry Double bond rule Trisilaallene

References

Illustrations

Worked examples

Example 1 — a first encounter with Silenes

Start with the simplest possible case. Write down what Silenes claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Silenes before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Silenes ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Silenes

In research: Silenes appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Silenes in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Silenes is common in secondary-school and first-year university syllabi. It links to neighbouring topics Silicon compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Silenes outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Silenes in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Silenes means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Silenes out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Silenes in simple terms?

In inorganic chemistry, silenes, or disilalkenes, are silicon compounds that contain Si=Si double bonds, where the oxidation state of Si is +2. The parent molecule is disilene, Si2H4.

Why does Silenes matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Silenes?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Silenes.