Lena Savoie
The solubility of a compound in water depends on its polarity. Polar substances are easily dissolved in water because polar solvents are hydrophilic. Alcohols are organic compounds that contain a hydroxy group attached to a carbon. The oxygen-hydrogen dipole in the hydroxy group facilitates hydrogen bonding with water, making alcohols soluble. However, phenols, a class of alcohols, have a hydroxy group attached to an aromatic ring, which affects their solubility in water.
| Characteristics | Values |
|---|---|
| Presence of benzene ring | Makes phenols non-polar and unable to form hydrogen bonds with water |
| Hydrogen bonding | Hydrogen in the hydroxyl group of phenol forms an intermolecular hydrogen bond with water molecules |
| Boiling point | Higher boiling point than aliphatic hydrocarbons of similar molecular weight due to intermolecular hydrogen bonding |
| Molecular weight | Alcohols with similar molecular weight have lower boiling points |
| Carbon chain length | Longer carbon chain length reduces solubility in water |
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What You'll Learn
The presence of the benzene ring
Additionally, the oxygen s electrons in the p orbital delocalize into the ring, reducing the ability of phenols to form hydrogen bonds with water. The hydrogen in the hydroxyl group (OH) of phenol can still form hydrogen bonds with water, but the oxygen is unable to do so due to the delocalization of electrons. This makes phenol only partially soluble in water.
The benzene ring is also hydrophobic, further contributing to the reduced solubility of phenols in water. The hydrophobic nature of the ring is a result of the delocalized electron system, which prevents the ring from forming strong electrostatic interactions with water.
Overall, the presence of the benzene ring in phenols reduces their solubility in water due to its non-polar and hydrophobic nature, as well as its impact on the ability of phenols to form hydrogen bonds with water molecules.
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The oxygen's electrons delocalise into the ring
The oxygen's electrons delocalize into the ring, reducing the ability of phenol to form hydrogen bonds with water. The oxygen in the -OH group donates its lone pair to form a pi bond, which is a type of resonance. This is possible due to the presence of the benzene ring, which consists of one electron from each carbon atom. The carbon atoms have four electrons each, so the delocalized ring does not steal electrons from the C-H bond. The overall dipoles cancel each other out, making the ring non-polar. Non-polar compounds cannot form hydrogen bonds with polar water molecules.
The benzene ring is also hydrophobic, which further contributes to the reduced solubility of phenol in water. The hydrogen in the OH group of phenol can still form hydrogen bonds, but the oxygen cannot due to the delocalization of electrons. This results in phenol being only partially soluble in water.
Phenol is an aromatic organic compound with the molecular formula C6H5OH, also known as carbolic acid, phenolic acid, or benzenol. It is a white crystalline solid that is volatile and flammable. Phenol is mildly acidic and requires careful handling to prevent chemical burns. It is considered a health hazard due to its acute toxicity.
The enhanced acidity of phenol compared to aliphatic alcohols is attributed to the resonance stabilization of the phenolate anion. The negative charge on oxygen is delocalized onto the ortho and para carbon atoms through the pi system. This delocalization of electrons on oxygen contributes to the reduced ability of phenol to form hydrogen bonds with water, making it less soluble.
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The ring is non-polar
The benzene ring in phenols is non-polar, which significantly contributes to the reduced solubility of phenols in water compared to other alcohols. This non-polar ring structure is a defining characteristic of the phenol molecule and sets it apart from other alcohol compounds.
The non-polar nature of the benzene ring arises from the unique arrangement of carbon and hydrogen atoms in the ring. The ring consists of six carbon atoms bonded together in a hexagonal structure, with hydrogen atoms attached to each carbon. This symmetric ring structure, with its
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The hydroxyl group is polar
The hydroxyl group, also known as an alcohol group, is a functional group with one hydrogen and one oxygen atom. This functional group is denoted as -OH and is covalently bonded to a carbon atom. The oxygen atom is significantly more electronegative than the hydrogen or carbon atoms it is bonded to, leading to an unequal distribution of electrons within the molecule. This unequal distribution results in the hydroxyl group exhibiting polar characteristics.
The polar nature of the hydroxyl group arises from the difference in electronegativity between the atoms it comprises. Oxygen is highly electronegative, attracting electrons towards itself and creating a partial negative charge. Conversely, carbon and hydrogen have relatively lower electronegativities, resulting in a partial positive charge on these atoms. This polarity is reflected in the hydroxyl group's ability to form hydrogen bonds with other molecules, particularly water.
The polarity of the hydroxyl group is a fundamental aspect of its reactivity and plays a crucial role in the characteristics of alcohols and phenols. The hydroxyl group's polar covalent bonds, specifically the O-H and O-C bonds, are responsible for the major reaction patterns observed in alcohols and phenols. These reactions typically occur through the interaction of electron-deficient or electron-rich groups with the negatively charged oxygen atom or the positively charged carbon or hydrogen atoms, respectively.
The hydroxyl group's polarity also contributes to the solubility of alcohols in water. Water molecules themselves contain hydroxyl groups, allowing them to form hydrogen bonds with other water molecules as well as with alcohol molecules. This ability to form hydrogen bonds between water and alcohol molecules leads to the relatively high solubility of alcohols in water.
Furthermore, the polarity of the hydroxyl group influences the behaviour of carboxylic acids, which are formed by the combination of a carbonyl group and a hydroxyl group attached to the same carbon atom. The hydroxyl group within carboxylic acids can release a hydrogen atom as a free proton (H+), leaving a delocalized negative charge on the remaining oxygen atom. This delocalized charge enables carboxylic acids to form hydrogen bonds with other polar compounds in their protonated state and ionic bonds with positively charged compounds in their deprotonated state.
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The solubility of phenol is due to intermolecular hydrogen bonding
The solubility of phenols and alcohols in water is due to the presence of hydrogen bonding. Alcohols are organic compounds in which a hydroxy group is attached to a saturated carbon. Phenols are a class of alcohols containing a hydroxy group attached to an aromatic ring. The hydroxyl group in phenol has the ability to form intermolecular hydrogen bonds with water. This is because the high electronegativity of oxygen relative to carbon and hydrogen leads to a partial negative charge on oxygen and partial positive charges on hydrogen and carbon. The opposite partial charges of oxygen–hydrogen bond dipoles in adjacent alcohol or phenol molecules attract each other in hydrogen-bonding interactions.
In an aqueous solution, alcohols and phenols form large networks of hydrogen bonds with water molecules, which enhances their water solubility. However, the solubility of phenol is limited due to the presence of a large hydrocarbon group. The benzene ring is non-polar as the overall dipoles cancel each other out. Non-polar parts of a compound cannot form hydrogen bonds with polar water molecules. The oxygen s electrons in the p orbital delocalise into the ring, reducing the phenols' ability to make hydrogen bonds.
The boiling points of alcohols increase with the size of the alkyl region due to the greater surface area for interactions through dispersion forces. The increased surface area of the nonpolar region, where solvation by water is unfavourable, results in a lower solubility of alcohols in water. An alcohol with branching in the chain is more water-soluble than its linear equivalent, since branching reduces the contact surface of the nonpolar region. Additional hydrogen bonding sites, such as the second hydroxyl group in diols, increase the boiling point and water solubility of alcohols.
The boiling points of alcohols and phenols are also influenced by hydrogen bonding due to the oxygen–hydrogen dipole in the hydroxy functional group and dispersion forces between alkyl or aryl regions of alcohol and phenol molecules. Alcohols possess a higher boiling point than aliphatic hydrocarbons of similar molecular weights due to intermolecular hydrogen bonding. As in hydrocarbons, the dispersion forces are the reason for the higher boiling point upon increasing carbon chain length. Hydrogen bonding between the hydroxy group and water facilitates the solubility of alcohols in water. However, water solubility also depends on the length of the alkyl or nonpolar region of the molecule.
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Frequently asked questions
Phenols are less soluble in water than other alcohols because the oxygen s electrons in the p orbital delocalise into the ring, reducing their ability to make hydrogen bonds. The solubility of a compound in water is dependent on its polarity, and the benzene ring in phenols is non-polar.
Hydrogen bonding between the hydroxy group and water facilitates the solubility of alcohols in water. The high electronegativity of oxygen relative to carbon and hydrogen leads to a partial negative charge on oxygen and partial positive charges on hydrogen and carbon. The opposite partial charges of oxygen-hydrogen bond dipoles attract each other in hydrogen-bonding interactions.
An increased surface area of the non-polar region results in lower solubility in water. This is because solvation by water is unfavourable in these regions. An example is butan-1-ol, which is more soluble in water than pentan-1-ol due to its smaller hydrophobic region.
