Rank the given compounds on their relative acidity.
Here was the order I thought it was but it turned out to be wrong. (i based it based off of sp being most acidic and sp3 being least acidic)
STRONGEST
HC(triple bond)C-CH3
H2C=CH2
CH3NH2
H2O
CH4--> WEAKEST
and then I looked up the actual pka values and got the following order but it was still wrong...
HC(triple bond)C-CH3 --> STRONGEST
CH3NH2 pka 10.58
H20 pka 15.74
H2C=CH2 pka 44
Ch4 pka 50 ---> WEAKEST
please help I am really confused
Thanks :)
The correct order is:
STRONGEST
HC(triple bond)C-CH3
CH3NH2
H2C=CH2
H2O
CH4--> WEAKEST
Determining the relative acidity of compounds can sometimes be tricky, as it depends on various factors. However, there are a few key factors that can help us determine the order of acidity.
First, let's consider the strength of the acid. Essentially, this refers to the ability of the compound's conjugate base to stabilize the negative charge. Compounds with more electronegative atoms or more resonance structures tend to have more stable conjugate bases and are therefore stronger acids.
Second, we need to analyze the acidity trends among different functional groups. For example, compounds with a carbon-carbon triple bond (HC≡C-R) are generally more acidic than compounds with a carbon-carbon double bond (HC=CH-R) or only carbon-carbon single bonds (R-CH2-R' or CH3-R). This trend is because the acidity increases with increasing s-character of the carbon atom to which the hydrogen is attached.
Now, let's revisit the given compounds and analyze their acidity:
1. HC≡C-CH3: This compound has a carbon-carbon triple bond and is the strongest acid among the given compounds. The acidity is increased by the high s-character of the carbon atom to which the acidic hydrogen is attached.
2. CH3NH2: This compound is a weak base, meaning it can accept a proton rather than donate it. Although it contains a nitrogen atom, nitrogen is not as electronegative as an oxygen atom, reducing its acidity compared to water.
3. H2O: Water is a stronger acid than CH3NH2 due to the higher electronegativity of oxygen, which stabilizes the negative charge better.
4. H2C=CH2: This compound consists of a carbon-carbon double bond, making it less acidic than compounds with a triple bond. The sp2 hybridization of the carbon atoms reduces the s-character and thus decreases acidity compared to HC≡C-CH3 or H2C≡C-H.
5. CH4: This compound has only carbon-carbon single bonds and lacks any electronegative atoms that could stabilize the negative charge, making it the weakest acid among the given compounds.
Therefore, the correct order of acidity from strongest to weakest is:
HC≡C-CH3 > CH3NH2 > H2O > H2C=CH2 > CH4
Keep in mind that these trends may not apply in all cases, as there can be additional factors influencing acidity. However, understanding the concept of electronegativity, resonance, and s-character allows us to make educated predictions about relative acidities.
The order you initially thought based on the hybridization of the central atom is not always a reliable predictor of acidity. Acidity is determined by the stability of the conjugate base formed after loss of a proton. Let's consider the pKa values to rank the compounds correctly in terms of their relative acidity:
1. H2O (pKa 15.74) - Water is more acidic than methane (CH4) because it can readily donate a proton to form the hydroxide ion (OH-). Water has weakly acidic properties due to the stability of the hydroxide ion.
2. CH3NH2 (pKa 10.58) - Methylamine is a weak base and has slight acidic characteristics. It can accept a proton from water (acting as an acid) to form the ammonium ion (CH3NH3+).
3. H2C=CH2 (pKa 44) - Ethylene, being a hydrocarbon, is much weaker in terms of acidity compared to the previous compounds. It has a slightly acidic character due to the presence of a pi bond, which can accept a proton.
4. HC≡C-CH3 (pKa < 25) - Acetylene is more acidic than the aforementioned compounds. The presence of a triple bond allows for easy loss of a proton, forming the acetylide ion.
5. CH4 (pKa > 50) - Methane is the weakest acid on the list. It does not readily donate a proton and forms a stable methane cation.
So, the correct order of the compounds from strongest to weakest acidity is:
HC≡C-CH3 > CH3NH2 > H2O > H2C=CH2 > CH4
Please note that the pKa values are approximate and can vary depending on the conditions and substituents.