Organic Chemistry chapter 5
Hydrocarbons:
Alkane: C–C (single bonds)
Alkene: C=C (double bond)
Alkyne: C≡C (triple bond)
Oxygen-containing groups:
Alcohol: R–OH
Ether: R–O–R'
Aldehyde: R–CHO
Ketone: R–CO–R'
Carboxylic Acid: R–COOH
Ester: R–COOR'
Nitrogen-containing groups:
Amine: R–NH₂
Amide: R–CONH₂
Nitrile: R–C≡N
Hydrocarbons
Alkane (C–C single bonds):
Example: Methane (CH₄)
The simplest alkane with only single bonds.
Alkene (C=C double bond):
Example: Ethene (C₂H₄)
A simple alkene with a double bond between two carbon atoms.
Alkyne (C≡C triple bond):
Example: Ethyne (C₂H₂) (commonly known as acetylene)
A molecule with a triple bond between two carbons.
Oxygen-Containing Functional Groups
Alcohol (-OH):
Example: Ethanol (C₂H₅OH)
Found in alcoholic beverages, with an -OH group attached to an ethyl chain.
Ether (R–O–R'):
Example: Dimethyl ether (CH₃OCH₃)
An ether with two methyl groups attached to an oxygen atom.
Aldehyde (R–CHO):
Example: Formaldehyde (CH₂O)
The simplest aldehyde, often used in preserving biological specimens.
Ketone (R–CO–R'):
Example: Acetone (CH₃COCH₃)
Commonly used as a solvent, with a carbonyl group between two methyl groups.
Carboxylic Acid (R–COOH):
Example: Acetic acid (CH₃COOH)
Found in vinegar, with a carboxyl group attached to a methyl group.
Ester (R–COOR'):
Example: Ethyl acetate (CH₃COOCH₂CH₃)
Used as a solvent in nail polish remover, with an ester group linking two carbon chains.
Nitrogen-Containing Functional Groups
Amine (R–NH₂):
Example: Methylamine (CH₃NH₂)
A simple amine with a methyl group attached to an amine (NH₂) group.
Amide (R–CONH₂):
Example: Acetamide (CH₃CONH₂)
An amide with a carbonyl group attached to a nitrogen atom and a methyl group.
Nitrile (R–C≡N):
Example: Acetonitrile (CH₃C≡N)
Contains a nitrile group with a triple bond between carbon and nitrogen
Alkyl Halides vs Alcohols
Alkyl halides and alcohols are both classes of organic compounds, but they differ in the functional groups attached to the carbon atoms.
Alkyl Halides (Haloalkanes):
These compounds contain a halogen atom (F, Cl, Br, or I) attached to an sp³ hybridized carbon atom of an alkyl group.
General Formula: R–X, where R is an alkyl group (like CH₃ or CH₃CH₂) and X is a halogen (F, Cl, Br, or I).
Example: Chloromethane (CH₃Cl) — a methyl group (CH₃) with a chlorine atom attached.
Alcohols:
Alcohols have a hydroxyl group (-OH) attached to a saturated carbon atom (sp³ hybridized).
General Formula: R–OH, where R is an alkyl group and OH is the hydroxyl group.
Example: Methanol (CH₃OH) — a methyl group (CH₃) with an -OH group attached.
The key difference is the functional group:
Alkyl halides have halogens (X), while alcohols have hydroxyl (-OH) groups.
Both groups can participate in nucleophilic substitution reactions, but the reactivity differs based on the nature of the attached functional group. Alcohols often undergo dehydration and oxidation reactions, while alkyl halides undergo substitution and elimination reactions.
Examples of IUPAC Nomenclature for Alkyl Halides
Chloromethane (CH₃Cl)
Parent Chain: Methane (CH₄)
Halogen Substituent: Chlorine (Cl)
IUPAC Name: Chloromethane (common name is methyl chloride).
1-Bromopropane (CH₃CH₂CH₂Br)
Parent Chain: Propane (CH₃CH₂CH₃)
Halogen Substituent: Bromine (Br) at position 1
IUPAC Name: 1-Bromopropane.
2-Chlorobutane (CH₃CH₂CHClCH₃)
Parent Chain: Butane (CH₃CH₂CH₂CH₃)
Halogen Substituent: Chlorine (Cl) at position 2
IUPAC Name: 2-Chlorobutane.
1,2-Dibromoethane (BrCH₂CH₂Br)
Parent Chain: Ethane (CH₃CH₃)
Halogen Substituents: Bromine (Br) at positions 1 and 2
IUPAC Name: 1,2-Dibromoethane (common name is ethylene dibromide).
2,2-Dichloropropane (CH₃CCl₂CH₃)
Parent Chain: Propane (CH₃CH₂CH₃)
Halogen Substituents: Two chlorine atoms at position 2
IUPAC Name: 2,2-Dichloropropane.
1. Substitutive Nomenclature
In substitutive nomenclature, the functional group is treated as a substituent on the main carbon chain. This is the method most commonly used in IUPAC nomenclature. The functional group is named as a prefix or suffix, and the longest continuous carbon chain is chosen as the parent structure.
Key Characteristics:
Functional groups are treated as part of the main molecule.
The functional group is named as a prefix (for halides, for example) or suffix (like -ol for alcohols).
You number the parent carbon chain to give the functional group the lowest possible position.
Example of Substitutive Nomenclature:
For Alkyl Halides:
1-Chlorobutane (CH₃CH₂CH₂CH₂Cl)
Here, "chlorine" is treated as a substituent, and the parent chain is "butane" (a 4-carbon alkane). Chlorine is located at the first carbon, so the name is 1-chlorobutane.
For Alcohols:
Butan-1-ol (CH₃CH₂CH₂CH₂OH)
Here, the -OH group is treated as a functional group attached to the main chain. The parent chain is "butane," and the -OH group is at position 1, so the name is butan-1-ol.
2. Functional Class Nomenclature
In functional class nomenclature (also known as radicofunctional nomenclature), the compound is named as a combination of a functional group and an alkyl group. The functional group is named separately, and the alkyl group is named as a substituent. This system is often used for simpler compounds or in more traditional contexts.
Key Characteristics:
The functional group is named as a separate entity from the alkyl chain.
The alkyl chain is treated as a separate substituent, and the name is written as a two-part name.
Typically used for simpler compounds or where the functional group is small, such as halides or alcohols.
Example of Functional Class Nomenclature:
For Alkyl Halides:
Butyl chloride (CH₃CH₂CH₂CH₂Cl)
Here, the name is split into two parts: "butyl" (the alkyl group) and "chloride" (the functional group). The functional group (chloride) is treated as a separate entity, resulting in the name butyl chloride.
For Alcohols:
Butyl alcohol (CH₃CH₂CH₂CH₂OH)
In this case, "butyl" represents the alkyl group, and "alcohol" refers to the hydroxyl functional group. This gives the name butyl alcohol.
Classes of Alcohols
Alcohols are classified based on the number of carbon atoms attached to the carbon atom bearing the hydroxyl group (-OH). There are three main classes of alcohols:
Primary Alcohols (1°)
The carbon atom bonded to the hydroxyl group (-OH) is attached to only one other carbon atom.
General formula: RCH₂OH
Example:
Ethanol (CH₃CH₂OH): The hydroxyl group is attached to a carbon that is bonded to only one other carbon.
Butan-1-ol (CH₃CH₂CH₂CH₂OH): The hydroxyl group is on the first carbon, which is only attached to one other carbon.
Secondary Alcohols (2°)
The carbon atom bonded to the hydroxyl group is attached to two other carbon atoms.
General formula: R₂CHOH
Example:
Propan-2-ol (CH₃CH(OH)CH₃): The hydroxyl group is on the second carbon, which is attached to two other carbons (one on either side).
Butan-2-ol (CH₃CH₂CH(OH)CH₃): The -OH group is attached to carbon 2, which is connected to two other carbon atoms.
Tertiary Alcohols (3°)
The carbon atom bonded to the hydroxyl group is attached to three other carbon atoms.
General formula: R₃COH
Example:
2-Methylpropan-2-ol (CH₃C(OH)(CH₃)₂): The hydroxyl group is attached to a carbon that is bonded to three other carbon atoms.
Tert-butyl alcohol (CH₃)₃COH: The -OH is attached to a carbon that is connected to three methyl groups.
Classes of Alkyl Halides
Alkyl halides (haloalkanes) are classified similarly to alcohols, based on the number of carbon atoms attached to the carbon that is bonded to the halogen atom (X = F, Cl, Br, I).
Primary Alkyl Halides (1°)
The carbon atom bonded to the halogen is attached to one other carbon atom.
General formula: RCH₂X
Example:
Chloromethane (CH₃Cl): The carbon bonded to chlorine is attached to only one other carbon atom.
1-Chlorobutane (CH₃CH₂CH₂CH₂Cl): The chlorine atom is attached to a primary carbon (the first carbon in the chain).
Secondary Alkyl Halides (2°)
The carbon atom bonded to the halogen is attached to two other carbon atoms.
General formula: R₂CHX
Example:
2-Chloropropane (CH₃CHClCH₃): The chlorine atom is attached to the second carbon, which is bonded to two other carbon atoms.
2-Bromobutane (CH₃CH₂CHBrCH₃): Bromine is attached to carbon 2, which is bonded to two other carbon atoms.
Tertiary Alkyl Halides (3°)
The carbon atom bonded to the halogen is attached to three other carbon atoms.
General formula: R₃CX
Example:
Tert-butyl chloride (C(CH₃)₃Cl): The chlorine atom is attached to a carbon that is bonded to three other carbon atoms (all methyl groups).
2-Chloro-2-methylpropane (CH₃CCl(CH₃)₂): The chlorine is attached to a tertiary carbon, which is connected to three other carbon atoms.
Van der Waals Forces and Boiling Points
Van der Waals forces are weak intermolecular forces that arise from the interactions between molecules. These forces are critical in determining the physical properties of substances, such as boiling points, melting points, and solubility. Here’s an overview of the different types of Van der Waals forces and their influence on boiling points:
Types of Van der Waals Forces
Dispersion Forces (London Forces)
Description:
These are the weakest intermolecular forces and occur due to the temporary fluctuations in electron distribution within molecules, creating temporary dipoles. All molecules exhibit dispersion forces, but they are particularly significant in nonpolar molecules.
Factors Affecting Strength:
Molecular size: Larger molecules have more electrons, leading to stronger dispersion forces.
Shape: Molecules with larger surface areas can have stronger dispersion forces due to increased contact area.
Dipole-Dipole Interactions
Description:
These forces occur between polar molecules, where the positive end of one polar molecule is attracted to the negative end of another.
Factors Affecting Strength:
Polarity: The more polar the molecule, the stronger the dipole-dipole interactions. This is often quantified by the dipole moment.
Hydrogen Bonding
Description:
A special type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F). Hydrogen bonding is stronger than regular dipole-dipole interactions.
Factors Affecting Strength:
Electronegativity: The greater the difference in electronegativity between hydrogen and the atom it is bonded to, the stronger the hydrogen bond.
Influence on Boiling Points
The boiling point of a substance is influenced by the strength of the intermolecular forces holding its molecules together. Here’s how different types of Van der Waals forces affect boiling points:
Dispersion Forces
Substances with only dispersion forces (like noble gases or nonpolar hydrocarbons) typically have lower boiling points due to the weak nature of these interactions.
Example:
Methane (CH₄) has a boiling point of -161.5 °C, while ethane (C₂H₆) has a boiling point of -88.6 °C. As the size of the molecule increases, so do the dispersion forces, leading to higher boiling points.
Dipole-Dipole Interactions
Polar molecules with significant dipole-dipole interactions will have higher boiling points than nonpolar molecules of similar molecular weight because the dipole-dipole interactions add to the overall intermolecular attractions.
Example:
Acetone (C₃H₆O), a polar molecule with a dipole moment, has a boiling point of 56 °C, which is higher than that of nonpolar compounds like hexane (C₆H₁₄), which has a boiling point of 68.7 °C.
Hydrogen Bonding
Molecules that can form hydrogen bonds typically have significantly higher boiling points than those that cannot, even if they have similar molecular weights. This is because hydrogen bonds are much stronger than both dispersion forces and dipole-dipole interactions.
Example:
Water (H₂O), which can form hydrogen bonds, has a boiling point of 100 °C, while a similar-sized nonpolar molecule, like hexane (C₆H₁₄), has a boiling point of about 68.7 °C.
Summary of Factors Affecting Boiling Points:
Molecular Size: Larger molecules tend to have higher boiling points due to increased dispersion forces.
Polarity: Polar molecules generally have higher boiling points than nonpolar molecules of similar size due to dipole-dipole interactions.
Hydrogen Bonding: Substances that can form hydrogen bonds have significantly higher boiling points than those that cannot.
Trends in Boiling Points:
Alkanes: Generally, as the chain length increases, boiling points increase due to stronger dispersion forces.
Alcohols: Alcohols have higher boiling points than alkanes due to hydrogen bonding.
Halides: Alkyl halides have intermediate boiling points, influenced by both dipole-dipole interactions and dispersion forces.
he SN1 and SN2 reactions are two distinct types of nucleophilic substitution reactions in organic chemistry. Here’s a detailed comparison between them, including their mechanisms, characteristics, and factors affecting them.
SN1 Reactions (Unimolecular Nucleophilic Substitution)
Mechanism:
Two-Step Process:
Step 1: Formation of a carbocation. The leaving group departs, creating a carbocation intermediate.
Step 2: Nucleophilic attack. The nucleophile attacks the carbocation, resulting in the formation of the product.
Characteristics:
Rate Determining Step: The first step (formation of the carbocation) is the slowest and determines the rate of the reaction. Therefore, the reaction rate depends only on the concentration of the substrate.
Reaction Rate Equation: Rate = k [substrate]
Carbocation Stability: The more stable the carbocation, the faster the reaction. Tertiary carbocations are most stable, followed by secondary, and then primary.
Stereochemistry: The reaction can lead to racemization (mixture of enantiomers) since the nucleophile can attack from either side of the planar carbocation.
Solvent: Favorable in polar protic solvents, which stabilize the carbocation and the leaving group.
Example:
The hydrolysis of tert-butyl chloride (C₄H₉Cl) in water: (CH₃)₃CCl→H₂O(CH₃)₃COH+HCl\text{(CH₃)₃CCl} \xrightarrow{\text{H₂O}} \text{(CH₃)₃COH} + \text{HCl}(CH₃)₃CClH₂O(CH₃)₃COH+HCl
SN2 Reactions (Bimolecular Nucleophilic Substitution)
Mechanism:
One-Step Process:
The nucleophile attacks the substrate at the same time as the leaving group departs, resulting in a concerted reaction.
Characteristics:
Rate Determining Step: The reaction occurs in a single step, and the rate depends on the concentrations of both the substrate and the nucleophile.
Reaction Rate Equation: Rate = k [substrate][nucleophile]
Steric Hindrance: Reactions occur more readily with primary substrates than with secondary or tertiary ones due to steric hindrance, which hinders the nucleophile's approach.
Stereochemistry: The reaction results in inversion of configuration at the chiral center (backside attack), following the Walden inversion principle.
Solvent: Favorable in polar aprotic solvents, which do not stabilize the nucleophile as much as polar protic solvents.
Example:
The reaction of sodium hydroxide with bromoethane: C₂H₅Br+NaOH→C₂H₅OH+NaBr\text{C₂H₅Br} + \text{NaOH} \rightarrow \text{C₂H₅OH} + \text{NaBr}C₂H₅Br+NaOH→C₂H₅OH+NaBr
Structure of Carbocations
Basic Structure:
A carbocation consists of a carbon atom that is bonded to three other atoms (either carbon or hydrogen) and has a positive charge due to a deficiency of one electron.
The carbon atom in a carbocation is sp² hybridized, leading to a trigonal planar geometry with bond angles of approximately 120°.
Lone Pair Absence:
Since carbocations have only three bonds and lack a full octet, they are electron-deficient and unstable compared to neutral carbon species.
Hybridization:
The sp² hybridization in carbocations means that one of the p orbitals remains unhybridized. This unhybridized p orbital can interact with neighboring atoms, which plays a role in the reactivity of carbocations.
Bonding in Carbocations
Bonding Configuration:
In a carbocation, the carbon atom forms three sigma (σ) bonds with surrounding atoms and has an empty p orbital, resulting in a total of four valence orbitals.
The empty p orbital can overlap with the orbitals of nucleophiles, allowing the carbocation to participate in further reactions.
Resonance:
Carbocations can often be stabilized through resonance, where the positive charge is delocalized across adjacent pi bonds or lone pairs.
For example, in allylic and benzylic carbocations, the positive charge can be shared between multiple atoms, increasing stability.
Stability of Carbocations
Carbocation stability follows a general trend based on the degree of substitution and resonance effects:
Degree of Substitution:
Tertiary Carbocations (3°): Most stable due to three alkyl groups donating electron density through hyperconjugation and inductive effects.
Example: (CH₃)₃C+\text{(CH₃)₃C}^+(CH₃)₃C+
Secondary Carbocations (2°): Less stable than tertiary but more stable than primary.
Example: (CH₃)₂CH+\text{(CH₃)₂CH}^+(CH₃)₂CH+
Primary Carbocations (1°): Least stable, with only one alkyl group providing stabilization.
Example: CH₃CH₂+\text{CH₃CH₂}^+CH₃CH₂+
Methyl Carbocation (0°): Extremely unstable, having no alkyl groups for stabilization.
Example: CH₃+\text{CH₃}^+CH₃+
Inductive Effect:
Alkyl groups are electron-donating due to their inductive effect, which helps stabilize the positive charge on the carbocation.
Resonance Stabilization:
Carbocations adjacent to double bonds or aromatic systems can be significantly stabilized due to resonance.
Example: The allylic carbocation C₃H₅+\text{C₃H₅}^+C₃H₅+ is stabilized through resonance with the double bond.
1. Naming Alcohols and Alkyl Halides Utilizing IUPAC
Example of Alcohol: 2-Propanol (Isopropyl alcohol, CH₃CHOHCH₃)
Example of Alkyl Halide: 1-Bromobutane (CH₃(CH₂)₃Br)
2. Evaluating Fundamental Properties of Alcohol and Alkyl Halide Solubility and Boiling Points
Solubility:
Alcohols (like ethanol) are soluble in water due to hydrogen bonding.
Alkyl halides (like bromoethane) are less soluble in water.
Boiling Point:
Ethanol (78.37 °C) has a higher boiling point than ethane (−88.6 °C) due to hydrogen bonding.
1-Bromobutane has a boiling point of 101.2 °C, which is higher than that of butane (−0.5 °C) due to the polar C-Br bond.
3. Broadly Describing the Process of Substitution Reactions in Organic Chemistry
Example: The reaction of ethanol with hydrochloric acid to form bromoethane (substitution of the -OH group with Cl): C₂H₅OH+HCl→C₂H₅Cl+H₂O\text{C₂H₅OH} + \text{HCl} \rightarrow \text{C₂H₅Cl} + \text{H₂O}C₂H₅OH+HCl→C₂H₅Cl+H₂O
4. Predicting Products of Alcohol→Alkyl Halide Substitution Reactions
Example: The reaction of 2-butanol with HBr: C₄H₉OH+HBr→C₄H₉Br+H₂O\text{C₄H₉OH} + \text{HBr} \rightarrow \text{C₄H₉Br} + \text{H₂O}C₄H₉OH+HBr→C₄H₉Br+H₂O
Product: 2-Bromobutane
5. Describing the SN1 Type of HX Substitution Reactions with Alcohols
Example: The reaction of tert-butanol with HCl: (CH₃)₃COH+HCl→(CH₃)₃CCl+H₂O\text{(CH₃)₃COH} + \text{HCl} \rightarrow \text{(CH₃)₃CCl} + \text{H₂O}(CH₃)₃COH+HCl→(CH₃)₃CCl+H₂O
Mechanism involves carbocation formation.
6. Chemically Expressing the SN1 Mechanism
Example: For the reaction of 2-butanol with HBr:
Step 1 (Ionization):
C₄H₉OH→C₄H₉++OH−\text{C₄H₉OH} \rightarrow \text{C₄H₉}^+ + \text{OH}^-C₄H₉OH→C₄H₉++OH−
Step 2 (Nucleophilic Attack):
C₄H₉++Br−→C₄H₉Br\text{C₄H₉}^+ + \text{Br}^- \rightarrow \text{C₄H₉Br}C₄H₉++Br−→C₄H₉Br
7. Labeling an SN1 Progress of Reaction vs. Energy Diagram
Example: The diagram would show the energy profile with:
Reactants (2-butanol) → Activation Energy → Carbocation (transition state) → Products (2-bromobutane) with an energy well for the carbocation intermediate.
8. Predicting Product and Commenting on the Acid-Assisted SN2 Mechanism
Example: The reaction of a primary alcohol (ethanol) with HBr: C₂H₅OH+HBr→C₂H₅Br+H₂O\text{C₂H₅OH} + \text{HBr} \rightarrow \text{C₂H₅Br} + \text{H₂O}C₂H₅OH+HBr→C₂H₅Br+H₂O
This reaction proceeds via an SN2 mechanism with inversion of configuration.
9. Evaluating the Relative Rates of Substitution Reactions of ROH Structure with HX
Example: The relative rates could be:
Tertiary alcohols (fastest) > Secondary alcohols > Primary alcohols (slowest) with HCl.
10. Properly Drawing a Carbocation Including its Empty p Orbital
Example: For a tertiary carbocation (tert-butyl cation): (CH₃)₃C+(with an empty p orbital shown)\text{(CH₃)₃C}^+ \quad (\text{with an empty p orbital shown})(CH₃)₃C+(with an empty p orbital shown)
11. Evaluating Stability Differences Among Different Carbocation Structures
Example:
Tertiary Carbocation: More stable due to hyperconjugation and inductive effects from three alkyl groups.
Secondary Carbocation: Less stable than tertiary but more stable than primary.
Primary Carbocation: Least stable, with only one alkyl group providing stabilization.
Methyl Carbocation: Highly unstable due to no stabilizing alkyl groups.
12. Describing and Showing How Inductive Donation Helps Stabilize a Carbocation
Example: In a tertiary carbocation, the electron-donating effect of three methyl groups increases stability by spreading the positive charge.
13. Describing and Showing How Hyperconjugation Helps Stabilize a Carbocation
Example: A tertiary carbocation (tert-butyl) is stabilized by hyperconjugation from adjacent C-H sigma bonds interacting with the empty p orbital.
14. Predicting the Products of Reaction of Alcohols with PBr3 and SOCl2
Example:
Reaction of 2-propanol with PBr3:
C₃H₇OH+PBr₃→C₃H₇Br+HBr+POBr₂\text{C₃H₇OH} + \text{PBr₃} \rightarrow \text{C₃H₇Br} + \text{HBr} + \text{POBr₂}C₃H₇OH+PBr₃→C₃H₇Br+HBr+POBr₂
Product: 2-Bromopropane
15. Predicting Products of Sulfonyl Chlorides + Base + Chiral Alcohols
Example:
The reaction of a chiral alcohol (e.g., (R)-2-butanol) with tosyl chloride (TsCl) followed by a base will yield the corresponding tosylate without affecting stereochemistry.
16. Describing the Difference Between a Reaction Intermediate and a Reaction Transition State
Example:
Intermediate: Carbocation (e.g., (CH₃)₃C+\text{(CH₃)₃C}^+(CH₃)₃C+) formed during SN1 reactions.
Transition State: A high-energy state during the reaction where bonds are partially broken/formed.
17. Predicting the Product of a Reaction of an Alcohol with Sulfonic Acids to Form −OMES and –OTS Substituted Compounds
Example:
The reaction of 2-butanol with tosyl chloride (TsCl) results in the formation of 2-tosylate (2-OTS):
C₄H₉OH+TsCl→C₄H₉OTs+HCl\text{C₄H₉OH} + \text{TsCl} \rightarrow \text{C₄H₉OTs} + \text{HCl}C₄H₉OH+TsCl→C₄H₉OTs+HCl
Chapter 5 selected end of chapter problems with answers and light explanations
Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have similar dipole moments. Why?
bond length can affect dipole moment, Therefore the lower electronegativity is somewhat made up for by the
longer bond.
Write structual formulas for all the isomers of (C3H8O). One of these is a gas at 25C? Which one? Why?
1-propanol, 2-propanol, ethylmethyl ether (does not have hydrogen bonding donor so likly to be room temp)
Which would you expect to be more stable: (CHh)3C+ or (CF3)3C+
Carbocations are stabilized by electron donation, either through inductive or hyperconjugative processes. The difference between these cations is that trifluoromethyl groups are strong electron withdrawing groups and would therefore destabilize any cation. The (CH3)3C+ is
the more stable carbocation.