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Key Highlights
- The removal of hydrogen atoms from alkane results in the formation of the alkyl group.
- The general representation of an alkyl group, R-X, is often denoted by the letter "R".
- They exist in three forms: Alkyl free radicals, Alkyl anions, and Alkyl cations.
- Alkanes are expressed with a general formula −Cn H2n+2 and the formula for the corresponding alkyl group is −Cn H2n+1.
- The naming convention of these groups is done by adding the suffix “yl”.
The alkyl group becomes more reactive than the alkane due to the absence of hydrogen.
- They are the substituent of saturated hydrocarbons.
- Since alkyl groups, such as methyl and ethyl, are weak electron donors, they stabilize neighboring carbocations.
| Table of Content |
Key Terms: Alkyl Group, Alkane, Hydrogen, Carbon, Hydrocarbon, Alkanes, Radicals
What is an Alkyl Group?
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An organic compound composed of only atoms of hydrogen and carbon arranged in the form of chains is called an alkyl functional group. They are nonpolar because hydrogen and carbon have almost comparable electronegativity.
Nomenclature of Alkyl Groups
The suffix "yl" is used to denote an alkyl group in place of the alkane's "ane" suffix. Alkyl groups form similar series.
| Number of Carbon Atoms | Related Alkane | Root Word | Alkyl Group Formed | Structure of Alkyl Group |
|---|---|---|---|---|
| 1 | Methane | Meth | Methyl | CH3– |
| 2 | Ethane | Eth | Ethyl | CH3–CH2– |
| 3 | Propane | Prop | Propyl | CH3-CH2–CH2– |
| 4 | Butane | But | Butyl | CH3-CH2–CH2–CH2– |
| 5 | Pentane | Pent | Pentyl | CH3-CH2–CH2–CH2–CH2– |
| 6 | Hexane | Hex | Hexyl | CH3-CH2–CH2–CH2–CH2–CH2– |
| 7 | Heptane | Hept | Heptyl | CH3-CH2–CH2–CH2–CH2–CH2–CH2– |
| 8 | Octane | Oct | Octyl | CH3-CH2–CH2–CH2–CH2–CH2–CH2–CH2– |
| 9 | Nonane | Non | Nonyl | CH3-CH2–CH2–CH2–CH2–CH2–CH2–CH2–CH2– |
| 10 | Decane | Dec | Decyl | CH3-CH2–CH2–CH2–CH2–CH2–CH2–CH2–CH2– |
Formation of Alkyl Groups
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The Alkyl group is formed by the removal of one hydrogen atom from the related alkane. Methyl (CH3-) and ethyl (C2H5-) are two examples of alkyl groups. The prefixes sec- and tert- are frequently used for indicating particular kinds of branching substituents.
Examples of the formation of an alkyl group
Here are some examples of the formation of alkyl groups from an alkane.
CH4 —> CH3
C2H6 —> C2H5
C3H8 —> C3H7 |
Alkyl Cations & Alkyl Anions
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Alkyl groups form two charged ions: Alkyl Cations & Alkyl Anions
- A positively charged alkyl group is called an alkyl cation. The positive charge on carbon represents the shortage of electrons. Therefore, alkyl cation is also called carbocations.
- Based on the attachment of alkyl groups, alkyl cation is categorized as Primary, secondary, and tertiary alkyl cations.
- The negatively charged alkyl group is called an alkyl anion. The negative charge represents the presence of extra electrons. Therefore, alkyl anions are also called carbanions.
Alkyl Free Radicals
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An alkyl free radical is an alkyl group with at least one unpaired electron in its outermost shell. Overall, the tertiary alkyl free radical is more stable than the primary and secondary forms.
Sample Questions
Ques. How are the alkyl groups represented? (2 marks)
Ans. Among the functional groups, the alkyl group is one in which both hydrogen and carbon atoms are present. An alkyl group can be expressed generally as CnH2n+1, where n is an integer or number.
Ques. What do you mean by aryl group? (2 marks)
Ans. An aryl group is formed when one hydrogen atom is taken out of a simple aromatic compound. Hydrocarbons are mainly found in the aromatic ring. The -yl suffix is added to the nomenclature of hydrocarbons, such as indolyl, thienyl, phenyl, etc.
Ques. What are alkyl halides? (2 marks)
Ans. Alkyl halides are structures formed when halogen atoms substitute the hydrogen atoms in an alkane. They are also called haloalkanes. Some common examples of alkyl halides are R-Cl (Alkyl Chloride), R-I (Alkyl Iodide), etc.
Ques. What are saturated hydrocarbons? (2 marks)
Ans. When there is only one bond between each atom of carbon in a hydrocarbon, it is said to be saturated. Saturated hydrocarbons are also known as alkanes. It is represented by a formula–CnH2n+2.
Ques. What are unsaturated hydrocarbons? (2 marks)
Ans. Unsaturated hydrocarbons are formed when two or more carbon atoms are joined by a double or triple bond.
- When two atoms share two pairs of electrons, a double bond is formed between them.
- When three pairs of electrons are shared between two atoms, a triple bond is formed between them.
Ques. What are aromatic hydrocarbons? (2 marks)
Ans. Aromatic hydrocarbons are cyclic molecules with unique characteristics and alternating positions of single and double bonds. Toluene and benzene are the two most common examples of aromatic hydrocarbons.
Ques. What are the important uses of alkanes? (3 marks)
Ans. There are various uses of alkanes in day-to-day life such as:
- Alkanes are the primary components of crude oil and many petroleum products.
- Propane and butane are utilized as liquefied gases.
- Anti-corrosive agents include alkanes with a large number of carbon atoms.
Ques. What is hybridization in chemistry? (2 marks)
Ans. Hybridization is described as the combining of orbitals of atoms with corresponding energy levels to produce an equal number of new hybrid orbitals. Three forms of hybridization exist in nature: sp3, sp2, and sp.
Ques. Which form of carbocation is the most stable? (2 marks)
Ans. The tertiary form of carbocation is the most stable carbocation. It is represented by (CH3)3 + C. The positive charge of the carbocation is stabilized by the presence of three alkyl groups.
Ques. Is oxygen present in alkyl groups? (1 mark)
Ans. An oxygen atom lies between two alkyl groups in the functional group, known as an ether. Their formula is R-O-R', where R and R' are the representations of the alkyl groups.
Ques. Are alkyl groups recognized as functional groups? (1 mark)
Ans. Yes, alkyl groups are also considered as a functional group. The structure of the alkyl group consists of only two atoms: carbon and hydrogen.
Ques. Is it possible for alkenes or alkynes to produce alkyl groups? (2 marks)
Ans. Alkanes can be changed into alkyl groups by removing one hydrogen atom. Alkyl groups can never be formed when a hydrogen atom is taken out of an alkene or an alkyne.
Ques. Answer the following questions:- (2 marks)
(a). Can alkyl groups donate electrons?
(b). Are alkyl compounds electrophilic?
Ans. (a). Yes, alkyl groups act as electron-donating groups and this helps in increasing the electron density of the other molecule.
(b).The alkyl group in the reaction acts as an electrophile due to a positive charge in the carbonyl group.
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