Chemistry-only: keep side groups when drawing a repeating unit
مُقدَّم| English | المعنى |
|---|---|
| side group/saɪd ɡruːp/ | مجموعة جانبية |
| repeating unit/rɪˈpiːtɪŋ ˈjuːnɪt/ | وحدة مكررة |
What would explain this observation?
- Propene has three carbon atoms, but only two belong to C=C. In poly(propene), those two form the repeating backbone while the third stays as a CH₃ side group 侧基.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Propene can be written CH₂=CH–CH₃. Addition polymerisation gives the repeating unit 重复单元 –CH₂–CH(CH₃)–. The double-bond carbons become neighbouring backbone carbons with single continuation bonds. The methyl side group remains attached to the same carbon; it is not cut off or automatically inserted as a third backbone position. Each repeating unit retains C₃H₆, the same atoms as the monomer, because no small-molecule by-product is formed.
- repeating unit: The smallest stated structural pattern repeated along the polymer chain; side group: An atom or group attached to the stated polymer backbone rather than continuing that backbone.
What happens to the CH₃ group when propene forms poly(propene)?
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Polymerisation opens only the C=C; the CH3 rides along as a side group on its backbone carbon, giving poly(propene) its alternating methyl pattern. Nothing is lost as methane or water in addition.
Match each technical term to its precise meaning.
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Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Trace the two C=C carbons first and label their attached atoms or groups before changing any bond. In the repeat, the CH₂ carbon bonds to two hydrogens and two backbone neighbours. The CH carbon bonds to one hydrogen, the CH₃ group and two backbone neighbours. Both have four bonds. A supplied CH₂=CHCl example similarly gives –CH₂–CHCl–: chlorine stays attached. This supplied structure is for transferring the model, not an extra list of commercial names to memorise.
- Draw a given alkene in a fixed orientation, circle the double-bond carbons and copy their side groups into the repeating unit. Replace the double bond with a single one, add continuation bonds and brackets, then check every atom and valency. For a reversed task, remove the continuation bonds and restore C=C between the corresponding backbone carbons. Equivalent reversed drawings describe the same connectivity; placement on the page is not a different polymer identity.
Which two habits make the investigation or model in this case more defensible?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: twelve propene-derived repeating units contain 12×3=36 carbon atoms in total. Twenty-four are backbone carbon atoms, while twelve are in methyl side groups. They also contain 12×6=72 hydrogen atoms in the stated units. Distinguishing backbone from total carbon prevents an incorrect claim that the three-carbon monomer must produce a three-carbon linear repeating backbone.
Sixteen propene-derived repeating units contain three carbon atoms each. Count all their carbon atoms. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Sixteen propene-derived repeating units contain three carbon atoms each. Count all their carbon atoms.
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The result is 48 atoms. Known: 16 propene-derived units, each containing 3 carbon atoms (2 backbone + 1 side group). Total = 16 x 3 = 48 carbon atoms - 32 backbone and 16 in methyl side groups.
Check the conclusion and its limits
- Do not change CH₃ into CH₂ while leaving it attached as a side group, add water as an addition product or keep the original C=C. A polymer drawing does not establish biodegradability, recycling availability or mechanical strength by itself. Those claims require separate property and life-cycle evidence. The monomer-to-repeat representation applies on both tiers.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every carbon in an alkene monomer must become part of the polymer backbone. This claim is false: Only the two carbons of the original C=C join the backbone. A monomer's other carbons - propene's CH3 - hang off the chain as side groups; they are not forced into the backbone.
Chemistry-only: keep side groups when drawing a repeating unit: Trace the two C=C carbons first and label their attached atoms or groups before changing any bond. In the repeat, the CH₂ carbon bonds to two hydrogens and two backbone neighbours. The CH carbon bonds to one hydrogen, the CH₃ group and two backbone neighbours. Both have four bonds. A supplied CH₂=CHCl example similarly gives –CH₂–CHCl–: chlorine stays attached. This supplied structure is for transferring the model, not an extra list of commercial names to memorise.
Every carbon in an alkene monomer must become part of the polymer backbone.
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Only the two carbons of the original C=C join the backbone. A monomer's other carbons - propene's CH3 - hang off the chain as side groups; they are not forced into the backbone.
The smallest stated structural pattern repeated along the polymer chain: write the technical term.
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repeating unit means The smallest stated structural pattern repeated along the polymer chain.