Question

An alkyl halide \(X\) reacts with sodium to form 3,8-dimethyldecane. What is \(X ?\) (a) CCC(C)CCCl (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) (c) CC(CCCCCl)C(C)(C)C

Short answer

The correct alkyl halide is (a) with the SMILES notation 'CCC(C)CCCl'.

Step-by-step solution

Understand the Wurtz reaction

The Wurtz reaction involves the reaction of an alkyl halide with sodium in the presence of dry ether to form an alkane. Two moles of alkyl halide react with two moles of sodium to produce a new alkane with the alkyl groups joined together and sodium halide as a by-product.

Analyze the possible products

For the given problem, the product is 3,8-dimethyldecane. This means we are looking for an alkyl halide that, upon reaction with another identical molecule, will yield this specific alkane. We should check each of the given options to confirm if they result in this particular alkane when doubled and joined.

Eliminate improbable options

Option (b), the linear alkyl halide with four carbons, cannot give the required product as it would result in an octane derivative without branching. The SMILES representations in option (a) and option (c) should be translated into their structural formulas to properly assess them.

Translate SMILES to structural formulas

The SMILES notation 'CCC(C)CCCl' corresponds to a 7-carbon structure with a methyl branch on the third carbon and a chlorine on the last carbon. The notation 'CC(CCCCCl)C(C)(C)C' corresponds to a 7-carbon chain with two methyl groups on the first carbon, one methyl group on the fourth carbon, and a chlorine on the sixth carbon.

Determine the correct alkyl halide

Since we are looking for an alkyl halide that produces 3,8-dimethyldecane, we need an alkyl halide with a potential for branching at the 3rd and 8th carbons when two such molecules join. The structural formula corresponding to 'CCC(C)CCCl' is the correct alkyl halide because when two such molecules are joined head-to-tail in a Wurtz reaction, the resulting alkane will be 3,8-dimethyldecane.

Key concepts

Alkyl Halide Reactions

Alkyl halides, also known as haloalkanes, are a group of chemical compounds characterized by an alkyl group attached to a halogen atom. These molecules play a pivotal role in organic synthesis due to their reactivity. For example, alkyl halides are commonly used in nucleophilic substitution reactions, where the halogen is replaced by another nucleophile. However, a unique class of reactions utilized for the growth of carbon chains involves the coupling of alkyl halides using metals like sodium. This process, known as the Wurtz reaction, is traditionally used to synthesize higher alkanes from simpler alkyl halides.

In the Wurtz reaction, two alkyl halides react in the presence of sodium, usually in dry ether, resulting in the formation of a new carbon-carbon bond, and thus a larger alkane. The reaction mechanism includes the formation of a radical intermediate followed by dimerization. Since this reaction doubles the original alkyl chain, understanding the original structure of the haloalkane is crucial to predict the resulting product. The process also generates sodium halide as a by-product, which is one of the factors that can help assess the reaction’s progress in a laboratory setting.

Synthesis of Alkanes

Alkanes, comprising solely of carbon and hydrogen atoms and characterized by single bonds between carbon atoms, are the simplest form of hydrocarbons. The synthesis of alkanes in a laboratory can be achieved through various methods, including hydrogenation of alkenes or the aforementioned Wurtz reaction. This reaction is particularly significant in the portfolio of an organic chemist, as it allows the formation of higher alkanes by connecting smaller carbon chains.

For instance, when an alkyl halide such as an appropriate haloalkane reacts with sodium in a Wurtz reaction, it can lead to the formation of a more complex alkane with a longer carbon chain. This technique is a fundamental tool for building complex organic molecules from simpler building blocks, which is an essential procedure in the synthesis of various chemicals, pharmaceuticals, and materials. It's important to carefully select the alkyl halides used in this reaction to ensure correct placement of any branches in the final alkane product, which may dictate the compound's properties and potential applications.

Understanding SMILES Notation

SMILES (Simplified Molecular Input Line Entry System) notation is a shorthand way to represent molecular structures in a text format. It is widely used in cheminformatics to communicate chemical information because it can be easily read and processed by computers. SMILES strings encode the structure of a molecule by specifying the sequence of atoms and the bonds between them. For example, a simple chain of carbons might be represented as 'CCCC,' whereas a chain with branching would show parentheses to indicate the position of the branches.

Translating SMILES into a structural formula involves understanding the semantics of the notation. Atoms are represented by their element symbols, single bonds are usually omitted (assumed by default), double and triple bonds are represented by '=' and '#', respectively, and branching is indicated by parentheses. An important aspect of SMILES is that it allows for the representation of stereochemistry, which is vital for understanding the physical and chemical behavior of molecules. Learning to read and write SMILES can greatly enhance a student's ability to conceptualize molecular structures and understand their reactivities in a digital environment.