17.2 Properties of Alcohols and Phenols
3 min read•may 7, 2024
Alcohols and phenols are crucial organic compounds with unique properties. Their , abilities, and reactivity shape their behavior in chemical reactions. Phenols are generally more acidic than alcohols due to of the .
Substituents on phenols can dramatically affect their acidity. increase acidity, while decrease it. These effects are most pronounced at ortho and para positions due to resonance. Understanding these properties is key to predicting reactions and applications.
Properties of Alcohols and Phenols
Acidity of alcohols vs phenols
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- Phenols have greater acidity compared to alcohols
- Phenols typically have values around 10 while alcohols have pKa values in the range of 16-18
- The phenoxide anion exhibits resonance stabilization which makes it a weaker base and consequently makes the a stronger acid
- Resonance structures allow for delocalization of the negative charge on the oxygen atom across the
- Electron-withdrawing groups attached to the phenol ring further enhance the stability of the phenoxide anion leading to increased acidity
- and demonstrate this effect
- Alcohols exhibit weak acidity
- The lacks resonance stabilization making it a stronger base and the a weaker acid in comparison to phenols
- Electron-withdrawing groups in close proximity to the can moderately increase acidity via
- (pKa ≈ 12.4) displays higher acidity compared to ethanol (pKa ≈ 16) due to the electron-withdrawing fluorine atoms
Hydrogen bonding in alcohol properties
- Alcohols have the ability to form
- The partially positive hydrogen atom of the hydroxyl group in one molecule is attracted to the partially negative oxygen atom of the hydroxyl group in another molecule
- Hydrogen bonding results in elevated for alcohols compared to alkanes with similar molecular weights
- Additional energy is necessary to overcome the intermolecular forces and vaporize the molecules
- Ethanol (bp 78.4°C) possesses a higher boiling point than pentane (bp 36.1°C) despite having a lower molecular weight due to the presence of hydrogen bonding
- of alcohols in water is enhanced by their ability to form hydrogen bonds
- Alcohols can form hydrogen bonds with surrounding water molecules enabling them to dissolve
- Short-chain alcohols containing up to 4 carbon atoms are with water while longer-chain alcohols exhibit limited solubility
Substituent effects on phenol acidity
- Electron-withdrawing groups (EWGs) attached to phenols increase their acidity
- EWGs stabilize the phenoxide anion through a combination of resonance and inductive effects
- have a more significant impact than inductive effects
- Substituents at the ortho and para positions can participate in resonance stabilization
- Substituents at the meta position can only exert inductive effects
- EWGs include functional groups such as -NO2, -CN, -CF3, -C(O)R, and -SO3H
- Electron-donating groups (EDGs) attached to phenols decrease their acidity
- EDGs destabilize the phenoxide anion through a combination of resonance and inductive effects
- Resonance effects have a more significant impact than inductive effects
- Substituents at the ortho and para positions can participate in resonance destabilization
- Substituents at the meta position can only exert inductive effects
- EDGs include functional groups such as -R, -OR, -OH, -NH2, and -NR2
- The of phenols is influenced by substituents, with EDGs generally increasing nucleophilicity
Reactions of Alcohols and Phenols
- reactions can occur in alcohols, leading to the formation of alkenes
- are common for alcohols, especially under acidic conditions
- Phenols can undergo reactions, which are influenced by the activating effect of the hydroxyl group
Key Terms to Review (43)
Acidity: Acidity refers to the ability of a substance to donate protons (H+ ions) or the concentration of H+ ions in a solution. It is a fundamental concept in organic chemistry that governs the properties and reactivity of various functional groups, including alcohols, phenols, and carboxylic acids.
Acidity constant, Ka: The acidity constant, Ka, measures the strength of an acid in a solution by quantifying its tendency to donate a proton (H+) to a base. It is expressed as the equilibrium constant for the dissociation of the acid into its conjugate base and a proton in water.
Alcohol: In the context of organic chemistry, an alcohol is an organic compound in which a hydroxyl group (-OH) is bonded to a saturated carbon atom. The general formula for a simple alcohol can be represented as CnH2n+1OH, where n is the number of carbon atoms.
Alcohol: Alcohols are a class of organic compounds characterized by the presence of a hydroxyl (-OH) functional group attached to a saturated carbon atom. They are widely used in various chemical reactions and have diverse applications in organic synthesis, pharmaceutical industry, and everyday life.
Alkoxide Anion: An alkoxide anion is a negatively charged species formed when an alcohol (R-OH) loses a proton, resulting in the formation of an R-O⁻ group. This anionic species is an important intermediate in many organic reactions involving alcohols.
Alkoxide ion, RO−: An alkoxide ion is the conjugate base of an alcohol, formed by the deprotonation of an alcohol, resulting in a negatively charged oxygen bonded to an alkyl group. It is represented by the formula RO−, where R represents an alkyl group.
Alkyl group: An alkyl group is a type of hydrocarbon chain that branches off from the main molecular structure in organic compounds, typically derived by removing one hydrogen atom from an alkane, giving it the general formula CnH2n+1. These groups are not stable on their own but can form strong covalent bonds with other atoms or groups, influencing the compound's physical and chemical properties.
Alkyl Group: An alkyl group is a hydrocarbon substituent derived from an alkane by the removal of a single hydrogen atom. Alkyl groups are commonly found in various organic chemistry topics, including substituent effects in electrophilic substitutions, the properties of alcohols and phenols, and the preparation of aldehydes and ketones.
Aromatic Ring: An aromatic ring is a cyclic structure of carbon atoms with a unique pattern of alternating single and double bonds, creating a delocalized system of $\pi$-electrons. This structural feature is found in many organic compounds and is central to understanding the properties and reactivity of aromatic heterocycles and phenols.
Boiling Points: Boiling point is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, and bubbles of vapor form inside the liquid. This property is crucial in understanding the behavior and characteristics of various organic compounds, including alkanes, alcohols, carboxylic acids, and amines.
Chromic Acid: Chromic acid, also known as chromium(VI) oxide, is a strong oxidizing agent commonly used in organic chemistry. It is a bright orange-red crystalline compound with the chemical formula CrO3, which can be used to oxidize various organic compounds.
Dehydration: Dehydration is a chemical process in which water is removed from a compound, typically resulting in the formation of a new compound with fewer hydrogen and oxygen atoms. This term is particularly relevant in the context of various organic reactions and transformations, where dehydration plays a crucial role in the preparation and interconversion of different functional groups.
Electron-Donating Groups: Electron-donating groups are functional groups or substituents that have the ability to donate or contribute electrons to a molecule, typically a benzene ring or other aromatic system. These groups can have a significant impact on the reactivity, stability, and properties of the molecule.
Electron-Withdrawing Groups: Electron-withdrawing groups are functional groups or substituents in a molecule that have a strong affinity for electrons, making them attractive to electrons. This property can significantly influence the reactivity, stability, and spectroscopic properties of the molecule.
Electrophilic aromatic substitution: Electrophilic aromatic substitution is a chemical reaction in which an atom, typically hydrogen, attached to an aromatic system, such as benzene, is replaced by an electrophile. This process preserves the aromaticity of the compound while introducing a functional group.
Electrophilic Aromatic Substitution: Electrophilic aromatic substitution is a fundamental organic reaction in which an electrophile (a species that is attracted to electrons) replaces a hydrogen atom on an aromatic ring, resulting in the formation of a new carbon-electrophile bond. This reaction is crucial in understanding the behavior and reactivity of aromatic compounds, which are prevalent in many organic molecules and have widespread applications.
Elimination reactions: Elimination reactions are a type of organic reaction where two atoms or groups are removed from a molecule, resulting in the formation of a double bond. These reactions often involve the loss of small molecules like water or hydrogen halides from larger organic molecules.
Elimination Reactions: Elimination reactions are a class of organic reactions where two atoms or groups are removed from a molecule, typically resulting in the formation of a new carbon-carbon double bond. These reactions are an important aspect of organic chemistry, as they allow for the conversion of various functional groups and the synthesis of alkenes and other unsaturated compounds.
Esterification: Esterification is a chemical reaction that involves the formation of an ester compound from the reaction between a carboxylic acid and an alcohol. This process is crucial in various areas of organic chemistry, including the properties of functional groups, polar reactions, the behavior of alcohols and phenols, the reactions of carboxylic acids, the chemistry of esters, and the reactions of monosaccharides.
Hydrogen Bonding: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative element, such as nitrogen, oxygen, or fluorine, experiences an attractive force with another nearby highly electronegative element. This intermolecular force is stronger than a typical dipole-dipole interaction and has a significant impact on the physical and chemical properties of many organic compounds.
Hydroxyl Group: The hydroxyl group (OH-) is a functional group consisting of an oxygen atom covalently bonded to a hydrogen atom. It is a key structural feature in many organic compounds, particularly alcohols and phenols, and plays a crucial role in their chemical properties and reactivity.
Inductive Effects: Inductive effects refer to the ability of substituents or functional groups to influence the distribution of electron density within a molecule through space. This phenomenon can have significant implications on the stability, reactivity, and orientation of various organic reactions.
Intermolecular Hydrogen Bonds: Intermolecular hydrogen bonds are attractive forces that form between a hydrogen atom covalently bonded to a highly electronegative element, such as oxygen or nitrogen, and another nearby highly electronegative element. These bonds occur between separate molecules and are responsible for many of the unique physical and chemical properties of compounds containing hydrogen, oxygen, and nitrogen.
IR Spectroscopy: IR spectroscopy is a technique that uses infrared radiation to identify and analyze the molecular structure of organic compounds. It provides information about the vibrational modes of chemical bonds, allowing for the identification of functional groups and the determination of the overall structure of a molecule.
IUPAC Nomenclature: IUPAC nomenclature is a standardized system for naming organic compounds, developed by the International Union of Pure and Applied Chemistry (IUPAC). It provides a consistent and unambiguous way to identify and communicate the structure of organic molecules.
Lucas Reagent: The Lucas reagent is a chemical test used to distinguish between primary, secondary, and tertiary alcohols. It is a mixture of zinc chloride (ZnCl2) in concentrated hydrochloric acid (HCl) that reacts differently with each type of alcohol, allowing for their identification.
Miscible: Miscible refers to the ability of two or more liquids to be mixed together and form a single homogeneous phase without any separation or layering. This property is crucial in understanding the behavior and interactions of various substances, particularly alcohols and phenols, in organic chemistry.
Nucleophilicity: Nucleophilicity refers to the ability of a species to donate electrons and form a covalent bond with an electrophilic center. It is a key concept in organic chemistry that governs the reactivity and selectivity of many important reactions, including substitution, addition, and elimination reactions.
Ortho-cresol: Ortho-cresol, also known as 2-methylphenol, is a phenolic compound that is a structural isomer of cresol. It is a colorless, crystalline solid with a characteristic phenolic odor and is used in the production of various chemicals and pharmaceuticals.
Oxidation: Oxidation is a fundamental chemical process in which a substance loses electrons, resulting in an increase in its oxidation state. This term is central to understanding various reactions and transformations in organic chemistry, from the hydration of alkenes to the oxidation of alcohols and aldehydes.
P-Nitrophenol: p-Nitrophenol is a phenol derivative with a nitro group (-NO2) substituent attached to the para position of the aromatic ring. It is an important organic compound with various applications and unique properties.
Pentachlorophenol: Pentachlorophenol is a synthetic organic compound that is classified as a phenol. It is a white crystalline solid that has a strong, unpleasant odor and is used as a pesticide, wood preservative, and disinfectant due to its antimicrobial properties.
Phenol: Phenol is an aromatic organic compound with a hydroxyl group (-OH) attached directly to a benzene ring. It is a key structural feature in many important organic molecules and plays a significant role in various chemical reactions and properties across several topics in organic chemistry.
Phenoxide Anion: The phenoxide anion is a negatively charged species formed when a phenol (an aromatic alcohol) loses a hydrogen atom from the hydroxyl group. This anionic species is an important intermediate in various organic reactions and has unique properties that distinguish it from other organic anions.
Phenoxide ion, ArO–: The phenoxide ion is the anionic form of a phenol molecule (ArOH), where the hydrogen (H) atom of the hydroxyl group (-OH) is replaced by a negative charge (-). It results from the deprotonation of phenol, making it more reactive in nucleophilic substitution reactions.
Phenoxide ion, ArO−: A phenoxide ion is a negative ion formed when the hydrogen atom in a phenol molecule is replaced by a metal, resulting in an oxygen atom bonded to an aromatic ring with a negative charge. It is highly reactive and is involved in various organic reactions, particularly in nucleophilic substitutions.
PKa: pKa, or the acid dissociation constant, is a measure of the strength of an acid in a solution. It represents the pH at which a particular acid is 50% dissociated into its conjugate base. This value is crucial in understanding the behavior and properties of acids, bases, and their reactions in organic chemistry.
Primary Alcohol: A primary alcohol is an organic compound containing a hydroxyl (-OH) functional group attached to a saturated carbon atom that is bonded to only one other carbon atom. Primary alcohols are an important class of organic compounds with diverse applications and are a key focus in the study of organic chemistry.
Resonance Effects: Resonance effects refer to the stabilizing or destabilizing influence that certain substituents can have on adjacent functional groups or atoms within a molecule. This phenomenon arises from the ability of certain atoms or groups to participate in the delocalization of electrons, which can significantly impact the reactivity and properties of the molecule.
Resonance Stabilization: Resonance stabilization is a phenomenon where the delocalization of electrons in a molecule or ion leads to a more stable configuration compared to a single Lewis structure. This concept is crucial in understanding the behavior and properties of various organic compounds, including their acidity, basicity, reactivity, and stability.
Solubility: Solubility is a fundamental property that describes the ability of a substance to dissolve in a solvent, forming a homogeneous solution. It is a crucial concept in understanding the behavior and interactions of various compounds, including alcohols and phenols.
Substituent Effects: Substituent effects refer to the influence that specific functional groups or atoms have on the chemical and physical properties of a molecule. These effects can significantly impact the reactivity, stability, and behavior of organic compounds in various contexts, including conformational analysis, electrophilic and nucleophilic substitutions, and acidity determination.
Trifluoroethanol: Trifluoroethanol is a fluorinated alcohol with the chemical formula CF3CH2OH. It is a colorless, volatile liquid that is widely used in organic chemistry due to its unique properties and applications related to the properties of alcohols and phenols.