Alcohol oxidation and spectral identification
Introduit| English | Sens |
|---|---|
| distillation/dɪstɪˈleɪʃn/ | distillation |
| aldehyde/ˈældɪhaɪd/ | aldéhyde |
| reflux/ˈriːflʌks/ | reflux |
| ketone/ˈketəʊn/ | cétone |
| anhydrous/ænˈhaɪdrəs/ | anhydre |
How can one alcohol give two different products?
- Primary propan-1-ol can give propanal or propanoic acid with acidified dichromate. Product removal changes how far oxidation proceeds.
- Choose distillation 蒸馏 for aldehyde 醛 or reflux 回流 with excess oxidant for acid.
Which conditions favour propanal from propan-1-ol?
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Removing aldehyde limits further oxidation. Reflux with excess oxidant favours acid; phosphoric acid favours dehydration.
Use the alcohol structure
- Primary alcohols can oxidise to aldehydes and acids; secondary alcohols to ketones 酮. Tertiary alcohols resist these stated conditions.
- For propan-2-ol: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O. Dichromate changes orange to green.
Propan-2-ol oxidises to propanone with the stated acidified dichromate conditions.
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It is secondary, so oxidation gives a ketone.
Explain purification rather than naming it
- Distillation removes volatile material; reflux returns condensed vapour to the reaction mixture.
- Solvent extraction separates immiscible layers. A suitable anhydrous 无水的 salt removes residual water before final distillation.
Propan-1-ol + [O] → propanal + H₂O; propan-1-ol + 2[O] → propanoic acid + H₂O. Distil the aldehyde as it forms or use excess oxidant under reflux for acid.
Name the heating technique that returns condensed vapour to the reaction flask.
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Reflux allows prolonged heating while limiting loss of volatile reactants/products.
Combine mass and IR evidence
- A molecular-ion m/z gives molecular mass for a singly charged ion; the largest peak need not be the molecular ion.
- Use provided IR data to identify possible C=O, O–H, N–H, C=C, C–H and C–X groups. A peak alone rarely identifies one structure.
Calculate Mr for C₃H₆O. Use C = 12, H = 1, O = 16.
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Mr = 3×12 + 6×1 + 16 = 58. Both propanal and propanone have this value, so mass alone does not distinguish them.
Reject a competing structure
- C₃H₆O with C=O and a positive warm Fehling’s test fits propanal. Propanone has the same formula and carbonyl but fails that test.
- An acid gives CO₂ with carbonate and has a very broad O–H absorption as well as C=O. Record independent evidence and its limits.
“Every carbonyl absorption proves an aldehyde” is false. Ketones and carboxylic acids also have C=O; combine other evidence.
Use formula, spectra, chemical tests and reaction conditions together; distinguish purity evidence from structural proof.
Which two observations support a carboxylic acid?
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The acid chemical test and two functional-group absorptions support the proposal together.