Textbook Question
Predict the products formed when m-cresol (m-methylphenol) reacts with
(c) bromine in CCl4 in the dark
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20. Phenols
Phenol Acidity
Problem 51
Textbook Question
Rationalize the fact that 1,4-dihydroxybenzene melts at a significantly higher temperature than 1,2-diydroxybenzene.
Verified step by step guidance1
Identify the two compounds: 1,4-dihydroxybenzene (para-dihydroxybenzene) and 1,2-dihydroxybenzene (ortho-dihydroxybenzene). Note their melting points: 172°C for 1,4-dihydroxybenzene and 105°C for 1,2-dihydroxybenzene.
Consider the molecular structure of each compound. In 1,4-dihydroxybenzene, the hydroxyl groups are positioned opposite each other on the benzene ring, while in 1,2-dihydroxybenzene, the hydroxyl groups are adjacent.
Analyze the potential for hydrogen bonding. In 1,4-dihydroxybenzene, the hydroxyl groups can form intermolecular hydrogen bonds with other molecules, leading to a more stable and tightly packed crystal lattice.
In contrast, 1,2-dihydroxybenzene can form intramolecular hydrogen bonds between the adjacent hydroxyl groups, which reduces the ability to form strong intermolecular hydrogen bonds, resulting in a less stable crystal lattice.
Conclude that the stronger intermolecular hydrogen bonding in 1,4-dihydroxybenzene leads to a higher melting point compared to 1,2-dihydroxybenzene, where intramolecular hydrogen bonding predominates.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydrogen Bonding
Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen, interacts with another electronegative atom. In the case of 1,4-dihydroxybenzene, the hydroxyl groups are positioned para to each other, allowing for effective hydrogen bonding between molecules, which significantly increases the melting point.
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Molecular Structure and Isomerism
Molecular structure refers to the arrangement of atoms within a molecule, which can greatly influence its physical properties. 1,4-Dihydroxybenzene (para) has a symmetrical structure that facilitates stronger intermolecular interactions compared to 1,2-dihydroxybenzene (ortho), where steric hindrance can disrupt effective packing and bonding, leading to a lower melting point.
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Melting Point and Intermolecular Forces
The melting point of a substance is the temperature at which it transitions from solid to liquid, heavily influenced by the strength of intermolecular forces. Stronger intermolecular forces, such as hydrogen bonds in 1,4-dihydroxybenzene, result in a higher melting point compared to weaker forces in 1,2-dihydroxybenzene, which experiences more steric hindrance and less effective packing.
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