chemistry

Trimethylaluminium

Trimethylaluminium 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 Trimethylaluminium rather than just read about it. In short: Trimethylaluminium or TMA is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2(CH3)6 (abbreviated as Al2Me6, where Me stands for methyl), as it exists as a dimer.

Key takeaways

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

Reference excerpt

Trimethylaluminium or TMA is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2(CH3)6 (abbreviated as Al2Me6, where Me stands for methyl), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industrially important compound, closely related to triethylaluminium.

Structure and bonding The structure and bonding in Al2R6 and diborane are analogous (R = alkyl). In Al2Me6, the Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively. The Al center is tetrahedral. The carbon atoms of the bridging methyl groups are each surrounded by five neighbors: three hydrogen atoms and two aluminium atoms. The methyl groups interchange readily intramolecularly. At higher temperatures, the dimer Al2Me6 cracks into monomeric AlMe3.

Synthesis TMA is prepared via a two-step process that can be summarized as follows:

2 Al + 6 CH3Cl + 6 Na → Al2(CH3)6 + 6 NaCl

Applications

Catalysis Starting with the invention of Ziegler-Natta catalysis, organoaluminium compounds have a prominent role in the production of polyolefins, such as polyethylene and polypropylene. Methylaluminoxane, which is produced from TMA, is an activator for many transition metal catalysts.

Semiconductor applications TMA is also used in semiconductor fabrication to deposit thin film, high-k dielectrics such as Al2O3 via the processes of chemical vapor deposition or atomic layer deposition. TMA is the preferred precursor for metalorganic vapour phase epitaxy (MOVPE) of aluminium-containing compound semiconductors, such as AlAs, AlN, AlP, AlSb, AlGaAs, AlInGaAs, AlInGaP, AlGaN, AlInGaN, AlInGaNP, etc. Criteria for TMA quality focus on (a) elemental impurities, (b) oxygenated and organic impurities.

Photovoltaic applications In deposition processes very similar to semiconductor processing, TMA is used to deposit thin film, low-k (non-absorbing) dielectric layer stacks with Al2O3 via the processes of chemical vapor deposition or atomic layer deposition. The Al2O3 provides excellent surface passivation of p-doped silicon surfaces. The Al2O3 layer is typically the bottom layer with multiple silicon nitride (SixNy) layers for capping.

Reactions

Hydrolysis and related protonolysis reactions Trimethylaluminium is hydrolyzed readily, even dangerously:

Al2Me6 + 3 H2O → Al2O3 + 6 CH4 Under controlled conditions, the reaction can be stopped to give methylaluminoxane:

AlMe3 + H2O → 1/n [AlMeO]n + 2 CH4 Alcoholysis and aminolysis reactions proceed comparably. For example, dimethylamine gives the dialuminium diamide dimer:

2 AlMe3 + 2 HNMe2 → [AlMe2NMe2]2 + 2 CH4

Reactions with metal chlorides TMA reacts with many metal halides to install alkyl groups. When combined with gallium trichloride, it gives trimethylgallium. Al2Me6 reacts with aluminium trichloride to give (AlMe2Cl)2. TMA/metal halide reactions have emerged as reagents in organic synthesis. Tebbe's reagent, which is used for the methylenation of esters and ketones, is prepared from TMA and titanocene dichloride. In combination with 20 to 100 mol % Cp2ZrCl2 (zirconocene dichloride), the (CH3)2Al−CH3 adds "across" alkynes to give vinyl aluminium species that are useful in organic synthesis in a reaction known as carboalumination.

Adducts As for other "electron-deficient" compounds, trimethylaluminium gives adducts R3N·AlMe3. The Lewis acid properties of AlMe3 have been quantified. The enthalpy data show that AlMe3 is a hard acid and its acid parameters in the ECW model are EA = 8.66 and CA = 3.68. These adducts, e.g. the complex with the tertiary amine DABCO, are safer to handle than TMA itself. The NASA ATREX mission (Anomalous Transport Rocket Experiment) employed the white smoke that TMA forms on air contact to study the high altitude jet stream.

Synthetic reagent TMA is a source of methyl nucleophiles, akin to methyl lithium, but less reactive. It reacts with ketones to give, after a hydrolytic workup, tertiary alcohols.

Safety Trimethylaluminium is pyrophoric, reacting violently with air and water, releasing gases which can spontaneously ignite. Violent reactions are also possible with acids, oxygen, alcohols, halogens and oxidizing agents. It may cause severe skin burns and serious eye damage.

References

External links

NASA ATREX mission article.

Illustrations

Worked examples

Example 1 — a first encounter with Trimethylaluminium

Start with the simplest possible case. Write down what Trimethylaluminium 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 Trimethylaluminium 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 Trimethylaluminium 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 Trimethylaluminium

In research: Trimethylaluminium 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 Trimethylaluminium 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: Trimethylaluminium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coordination complexes, Dimers (chemistry), Four-membered rings, so understanding it makes those chapters shorter.
In everyday life: Look for Trimethylaluminium 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 Trimethylaluminium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Trimethylaluminium 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 Trimethylaluminium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Trimethylaluminium in simple terms?

Why does Trimethylaluminium 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 Trimethylaluminium?

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 Trimethylaluminium.