How many structural isomers are possible for C5H12?
To determine the number of structural isomers for a given molecular formula, we need to draw all possible unique arrangements of atoms while maintaining the connectivity and valency rules. For alkanes, structural isomers primarily involve changes in the carbon chain structure (straight chain, branched chain).
Identify the molecular formula: The given molecular formula is C₅H₁₂. This corresponds to the general formula for alkanes, C₂H₂₂ + 2, where \(n=5\).
Draw the straight-chain isomer: The simplest arrangement is a straight chain of five carbon atoms. This is n-pentane.
\[ \text{CH}_3 - \text{CH}_2 - \text{CH}_2 - \text{CH}_2 - \text{CH}_3 \]
Name: n-pentane
Draw branched-chain isomers with a four-carbon main chain: Reduce the main carbon chain to four carbons and place the remaining one carbon as a methyl branch.
\[ \text{CH}_3 - \text{CH}(\text{CH}_3) - \text{CH}_2 - \text{CH}_3 \]
Name: 2-methylbutane (also known as isopentane)
Note: Placing the methyl group on the second carbon from either end results in the same compound (2-methylbutane). Placing it on the first or fourth carbon would just extend the main chain back to five carbons (n-pentane).
Draw branched-chain isomers with a three-carbon main chain: Reduce the main carbon chain to three carbons. The remaining two carbons must be placed as branches.
\[ \text{CH}_3 - \text{C}(\text{CH}_3)_2 - \text{CH}_3 \]
Name: 2,2-dimethylpropane (also known as neopentane)
Note: There is only one way to attach two methyl groups to the central carbon of a three-carbon chain without forming a longer chain.
Check for uniqueness: We have identified three distinct structures:
These are all the possible structural isomers for C₅H₁₂.
C) 3. There are three structural isomers possible for C₅H₁₂: n-pentane, 2-methylbutane, and 2,2-dimethylpropane.
This question asks to identify the common name "neopentane" with its IUPAC name. Neopentane is a specific isomer of pentane. We need to draw the structures for each given IUPAC name and compare them to the structure of neopentane.
Understand Neopentane: Neopentane is a common name for an alkane with five carbon atoms (pentane) that has a highly branched structure. Specifically, it has a central carbon atom bonded to four methyl groups. Its structure is:
\[ \begin{array}{c} \text{CH}_3 \\ \text{ |} \\ \text{H}_3\text{C—C—CH}_3 \\ \text{ |} \\ \text{CH}_3 \end{array} \]
Determine the IUPAC name for Neopentane:
Evaluate the options:
A) 2-Methylbutane:
Longest chain: 4 carbons (butane).
Methyl group at position 2.
Structure: CH₃-CH(CH₃)-CH₂-CH₃. This is an isomer of pentane, also known as isopentane.
B) 3-Methylpentane:
Longest chain: 5 carbons (pentane).
Methyl group at position 3.
Structure: CH₃-CH₂-CH(CH₃)-CH₂-CH₃. This is a different isomer of hexane, not pentane (it has 6 carbons).
C) 2,2-Dimethylpropane:
Longest chain: 3 carbons (propane).
Two methyl groups at position 2.
Structure: CH₃-C(CH₃)₂-CH₃. This matches the structure and IUPAC name we derived for neopentane.
D) n-Pentane:
This is the straight-chain isomer of pentane.
Structure: CH₃-CH₂-CH₂-CH₂-CH₃.
C) 2,2-Dimethylpropane is the IUPAC name for neopentane. Both names refer to the same highly branched isomer of pentane.
A) 2-Methylbutane: This is the IUPAC name for isopentane, which is a different isomer of pentane. It has a four-carbon main chain with one methyl branch.
B) 3-Methylpentane: This compound has six carbon atoms in total (a five-carbon main chain and one methyl group), making it an isomer of hexane, not pentane. Therefore, it cannot be neopentane.
D) n-Pentane: This is the straight-chain isomer of pentane, which has no branching. Neopentane is a highly branched isomer.
The question asks for an alternative name for alkanes. This is a fundamental concept in organic chemistry, specifically hydrocarbon nomenclature. We need to identify the common name associated with the alkane family.
A) Paraffins — Alkanes are saturated hydrocarbons with single bonds. They are also known as paraffins, a term derived from their low reactivity ("parum affinis").
The question asks about the typical reaction type for alkanes. To answer this, we need to recall the chemical properties and reactivity of alkanes, which are saturated hydrocarbons.
C) Substitution reaction — Alkanes are saturated hydrocarbons, meaning they contain only single C-C and C-H bonds. They do not have double or triple bonds to undergo addition reactions. Their primary mode of reaction is the replacement of a hydrogen atom by another atom or group, typically through a free-radical mechanism, such as halogenation (e.g., methane reacting with chlorine in the presence of UV light to form chloromethane and HCl).
What is the general molecular formula of alkenes?
The question asks for the general molecular formula of alkenes. To determine this, we need to understand the basic structure of alkenes and how the number of carbon atoms relates to the number of hydrogen atoms in these compounds.
The hybridisation of carbon atoms involved in C=C double bond is:
To determine the hybridization of carbon atoms involved in a C=C double bond, we need to count the number of sigma bonds and lone pairs around each carbon atom. This count, known as the steric number, helps in identifying the hybridization state.
D) sp2 — Each carbon atom in a C=C double bond forms three sigma bonds (one with the other carbon and two with other atoms like hydrogen) and no lone pairs. This gives a steric number of 3, which corresponds to sp2 hybridization. The remaining unhybridized p-orbital on each carbon forms the pi bond.