Organic compounds are the class of compounds in which carbon atom or atoms are connected to other atoms, mostly hydrogen, oxygen, and nitrogen, through covalent bonds. Carbides, cyanides, and carbonates are the few carbon-containing non-organic compounds. Carbohydrates, lipids, proteins, and nucleotides are the organic compounds essential to human beings. The organic compounds having only carbon and hydrogen atoms, singly bonded with each other, are called alkanes. The general formula of alkanes is CnH2n+2. When any of the hydrogen from any aliphatic hydrocarbon chain is replaced by any halo group, the resulting compound is called alkyl halide. Organometallic compounds are compounds in which a metal atom replaces one hydrogen atom from any carbon.
Why do students struggle to learn alkyl halides, alkanes, and organometallics?
1. Lack of basic knowledge
Before going for any topic, the basics of the topic must be cleared. If you have to teach the organic compound, you must clear the concepts of basic organic chemistry first. Students assume every carbon and hydrogen-containing compound is an organic compound, which is not true. They don't know about the functional groups, attacking and leaving groups in any compound. Before going further, you must clarify what saturated and unsaturated organic compounds are. They also face difficulties when they have to identify any organic compound by the IUPAC name.
2. A large number of reactions and reaction mechanism
In organic chemistry, there is more than one possible reaction and reaction mechanism. Students face difficulties selecting the major pathway for the compounds, so you must be clear when there is more than one functional group, what the major product will be, and which mechanism it will follow. If there is more than one attacking and leaving group, students get confused about which group will attack and which one is the good leaving group. These terms must be cleared first to study any organic compound and its reactions.
3. It’s conceptually challenging
Theoretical studies of organic compounds and reactions are tricky because most compounds have 3D structures. Organic reactions involve complex reactions mechanisms that are difficult to understand theoretically. You can use 3D models of organic molecules so that students easily understand the structures of molecules. For reactions, mechanisms must use video graphics; if possible, draw the complete mechanism along with arrows identifying the reaction sites and reacting groups.
5 ways to make alkyl halides, alkanes, and organometallics interesting for students
1. Review the basics of organic chemistry
Before going to organic compounds and their reactions following terms must be cleared first:
An attacking group is a part of a molecule that attacks a substrate or intermediate to produce the product. In organic reactions, it may be an electrophile or a nucleophile. Electrophiles are the attacking groups that make chemical bonds with nucleophiles by accepting the electron pair. In contrast, nucleophiles make chemical bonds by donating the electron pair to the substrate or the intermediate.
A leaving group is a part of a molecule that can break away (leave the molecule) during a reaction. The key factor contributing to a species' suitedness as leaving group is its basicity: The weaker the base, the better the leaving group. Halogens are often used as the leaving group, e.g., in alkyl halides. The general order of "ability to leave" for them is I > Br > Cl > F.
Steric effects are non-bonding molecular interactions that influence the shape and reactivity of ions and molecules. Steric effects resulting from repulsive forces between functional groups arise from overlapping electron clouds. Steric hindrance refers to slowing a chemical reaction rate due to steric bulk interactions. Steric hindrance usually refers to the interaction of intermolecular groups. Understanding steric hindrance can allow the design of chemical reactions to force regioselectivity or stereoselectivity in a reaction or to minimize undesirable side reactions.
2. Explain Alkanes
Alkanes are the simplest structurally and least reactive hydrocarbon species containing only carbon and hydrogen. The distinguishing feature of alkanes is the lack of double or triple bonds - they are saturated hydrocarbons. The general formula of alkanes is CnH2n+2.
Saturated hydrocarbon compounds only contain single C-C and C-H sigma bonds. A significant amount of energy is contained within these strong bonds, meaning the oxidation of alkanes can produce huge amounts of heat. This makes alkanes excellent fuels, with their most significant reaction being combustion.
Figure 1: Structure of simplest alkanes (Methane and Ethane)
Alkanes are relatively unreactive but do undergo a few reactions. Alkanes and other hydrocarbons burn in the air to produce carbon dioxide and water to release heat in a combustion reaction. Nothing happens when alkanes are mixed with oxygen at room temperature, but with the introduction of a spark or flame, a vigorous combustion reaction takes place. Since hydrocarbon combustion is highly exothermic, they are used widely as fuels.
CH₄ + 2O₂ → 2CO + 2H₂O + heat
Figure 2. Combustion equation
In addition to combustion, alkanes can react with themselves in the presence of a catalyst and undergo an isomerization reaction to convert unbranched alkanes to their branched-chain isomers. Alkanes can also react with halogens such as bromine and chlorine but require increased temperature or UV light energy to initiate the reactions.
Figure 3: Image shows chlorination of methane (Figure source Labster theory)
3. Explain Alkyl halides
Alkyl halides (also known as haloalkanes) are hydrocarbon compounds in which one or more of the hydrogen atoms have been replaced by a halogen atom (iodine, bromine, chlorine, or fluorine). Incorporating halogen atoms into a hydrocarbon changes the compounds' physical properties, affecting size, electronegativity, bond length, and strength.
Alkyl halides are ideal substrates for reactions that require an excellent leaving group. The high reactivity of alkyl halides can be explained in terms of the nature of the C-X bond. The differences in electronegativity between the carbon and halogen atoms create a highly polarized bond resulting in a slightly electropositive carbon and slightly electronegative halogen.
Alkyl halides are classified according to the connectivity of the carbon atom that carries the halogen atom:
Primary alkyl hades are the alkyl halides on which the carbon attached to the halogen is only attached to one other alkyl group.
Secondary alkyl hades are the alkyl halides on which the carbon attached to the halogen is attached to two other alkyl groups.
Tertiary alkyl hades are the alkyl halides on which the carbon attached to the halogen is attached to three other alkyl groups.
This electron-deficient carbon on alkyl halides becomes a hotspot for a nucleophilic attack, making alkyl halides excellent substrates for nucleophilic substitution and elimination reactions. In general - due to the steric bulk of three alkyl groups surrounding the halogen in tertiary alkyl halides - tertiary alkyl halides are far less reactive than the other classes and may only participate in elimination reactions.
The general reactivity trend across alkyl halide classes is Primary > Secondary > Tertiary. However - this alkyl halide reactivity trend is reversed if the rate of a specific reaction (e.g., SN1 reaction) is determined by the formation of the most stable carbocation. In these situations, tertiary alkyl halides are highly favored as they would form the most stable reactive intermediate.
4. Explain organometallics
Organometallic reagents are chemical compounds that contain carbon-metal bonds. When the metal involved is Lithium, we can also refer to it as an organolithium reagent; when the metal is Magnesium, they are known as Grignard reagents. Organometallic reagents provide highly nucleophilic carbon sources, making them excellent starting materials in organic chemistry. They are one of the most synthetically valuable reagents for installing new carbon-carbon bonds.
The Grignard reaction is an organometallic chemical reaction in which an organomagnesium halide (also known as a Grignard reagent) adds to the carbonyl group of an aldehyde or ketone to form an alcohol. The Grignard reaction is one of the most important synthetic methods for forming carbon-carbon bonds.
Being extremely good nucleophiles, Grignard reagents are highly reactive compounds. This means that we would prepare these in situ in the lab just before we carry out our Grignard reaction. It also means we need to keep any trace of moisture out of our reaction. All glassware and solvents must be anhydrous (dry), and the reaction must be kept in a closed system where water and air cannot get in.
Figure 4: Grignard reagent formation reaction scheme (Figure source Labster theory)
Once the Grignard reagent has been prepared, we can add a solution of our carbonyl compound (aldehyde or ketone) to the reagent to perform the Grignard addition reaction.
Figure 5: Grignard addition reaction scheme (Figure source Labster theory)
The final work-up step involves pouring the reaction mixture into a mixture of sulfuric acid and ice to break down the Grignard transition complex and produce our alcohol product.
5. Use virtual lab simulations
With technological advances, it is much easier to explain complex and challenging processes with the help of simulations. Now, you can simulate experiments without the need for any valuable equipment.
In this regard, you can take help from Labster’s virtual lab simulations. These simulations engage students through interactive learning scenarios. Students dive into a 3D world, where they visually learn and apply their concepts to solving real-life problems.
In Labster’s interactive Reactions and Structure: Alkanes, alkyl halides, and organometallics Virtual Lab, students will learn the structures and reactions of alkanes and alkyl halides, compounds widely used in fossil fuels and refrigerants.