Chemistry Problem Solver

To calculate the molar mass of H2O (water), we need to sum the molar masses of each element in the compound based on its chemical formula.

The chemical formula for water is H2O, which means it contains 2 hydrogen atoms (H) and 1 oxygen atom (O).

Step 1: Find the atomic masses of hydrogen and oxygen

The atomic masses can be found on the periodic table:

• The atomic mass of hydrogen (H) is approximately 1.01 g/mol.
• The atomic mass of oxygen (O) is approximately 16.00 g/mol.

Step 2: Calculate the molar mass of H2O

Since there are 2 hydrogen atoms and 1 oxygen atom in water, the molar mass of H2O can be calculated as follows:

[ \text{Molar mass of H2O} = (2 \times \text{Atomic mass of H}) + (1 \times \text{Atomic mass of O}) ]

[ \text{Molar mass of H2O} = (2 \times 1.01 , \text{g/mol}) + (1 \times 16.00 , \text{g/mol}) ]

[ \text{Molar mass of H2O} = 2.02 , \text{g/mol} + 16.00 , \text{g/mol} ]

[ \text{Molar mass of H2O} = 18.02 , \text{g/mol} ]

Conclusion

The molar mass of H2O (water) is 18.02 g/mol. This value is essential for various calculations in chemistry, such as determining the number of moles in a given mass of water or the mass of water in a given number of moles.

Answer: c) Buchwald–Hartwig Reaction

1. Statement of the Reaction

Copy

Here Ar = an aryl (or heteroaryl) group; R₂NH = primary or secondary amine (R₂ = alkyl or aryl, R₃ = H, alkyl, or aryl).

2. Why it is the Buchwald–Hartwig Reaction

1. Coupling Partners - Aryl (pseudo)halide (Cl, Br, I, OTf) - Amines (R₂NH, primary or secondary)

2. Catalyst System - Palladium catalyst (e.g. Pd₂(dba)₃, Pd(OAc)₂) - Bulky, electron-rich phosphine ligands (e.g. BINAP, Xantphos, t-Bu₃P) - Base (e.g. NaOtBu, K₃PO₄, Cs₂CO₃)

3. Type of Bond Formed - C–N bond (aryl–amine)

4. Mechanistic Outline 1. Oxidative Addition Pd(0) + Ar–X → Ar–Pd(II)–X 2. Amine Coordination & Deprotonation Ar–Pd(II)–X + R₂NH → Ar–Pd(II)–NHR₂ + HX (base removes HX) 3. Reductive Elimination Ar–Pd(II)–NHR₂ → Ar–NHR₂ + Pd(0) (catalyst regenerated)

This three-step cycle (oxidative addition → amine coordination/deprotonation → reductive elimination) is the hallmark of the Buchwald–Hartwig amination.

3. Why the Other Options Do Not Match

• a) Ullmann Reaction - Usually copper-mediated, often high temperature, less tolerant of functional groups; generally older C–N couplings.

• b) Gabriel Phthalimide Synthesis - Converts alkyl halides into primary amines via phthalimide (no aryl–amine bond formation).

• d) Chan–Lam Coupling - Copper-mediated coupling of boronic acids (Ar–B(OH)₂) with amines or alcohols under aerobic conditions. Does not use Pd catalyst or aryl halide.

4. Conclusion

The palladium‐catalyzed cross‐coupling of (hetero)aryl halides with amines to form C–N bonds is known as the Buchwald–Hartwig Reaction.