Without looking them up, rank the following compounds in decreasing order of acidity. These examples represent large classes of compounds that differ widely in acidity. water, ethanol, 2-chloroethanol, tert-butyl alcohol, ammonia, sulfuric acid, hexane, hex-1-yne, acetic acid
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Identify the functional groups present in each compound and their general acidic behavior. For example, sulfuric acid is a strong acid, while hexane is a non-polar alkane and generally non-acidic.
Consider the effect of electronegativity and resonance on acidity. Electronegative atoms like chlorine in 2-chloroethanol can increase acidity by stabilizing the conjugate base.
Evaluate the hybridization of the carbon atoms adjacent to acidic hydrogens. For instance, the sp hybridization in hex-1-yne can increase acidity compared to sp3 hybridization in alkanes.
Assess the inductive effect and steric hindrance. For example, tert-butyl alcohol has steric hindrance that can decrease acidity compared to ethanol.
Rank the compounds based on the above considerations, starting with the strongest acid (sulfuric acid) and ending with the weakest (hexane).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acidity and pKa
Acidity refers to the ability of a compound to donate a proton (H+) in a chemical reaction. The strength of an acid is often measured by its pKa value, where lower pKa values indicate stronger acids. Understanding the pKa values of common compounds helps in ranking their acidity, as it provides a quantitative measure of their proton-donating ability.
Different functional groups in organic compounds significantly affect their acidity. For instance, carboxylic acids (like acetic acid) are generally more acidic than alcohols (like ethanol) due to the resonance stabilization of their conjugate bases. Recognizing how functional groups influence acidity is crucial for comparing the acidity of various compounds.
The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents on a molecule, which can influence acidity. Electronegative atoms, such as chlorine in 2-chloroethanol, can stabilize the negative charge of the conjugate base, enhancing acidity. Understanding these effects is essential for accurately ranking compounds based on their acidity.