Alkanes: Structure, Nomenclature and Physical Properties: 31 flashcards
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1. What are alkanes?
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Saturated hydrocarbons containing only carbon and hydrogen, with only single covalent bonds.
2. What is the general formula of alkanes?
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$C_nH_{2n+2}$, where $n = 1, 2, 3, \dots$
3. Why are alkanes called paraffins?
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From Latin parum (little) + affinis (affinity), because they are highly inert toward most reagents.
4. What is the hybridisation of carbon in alkanes?
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$sp^3$ hybridised, giving tetrahedral geometry with bond angles of about $109.5^\circ$.
5. What type of bonds do alkanes contain?
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Only sigma ($\sigma$) covalent bonds — C–C and C–H single bonds.
6. Name the three parts of an IUPAC name.
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Prefix(es), root word, and suffix(es).
7. What does the root word indicate?
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The number of carbon atoms in the longest carbon chain (parent chain).
8. List the root words for $C_1$ to $C_{10}$.
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Meth, Eth, Prop, But, Pent, Hex, Hept, Oct, Non, Dec.
9. What do the primary suffixes -ane, -ene and -yne indicate?
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-ane = single bond (alkane), -ene = double bond (alkene), -yne = triple bond (alkyne).
10. What is a parent chain?
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The longest continuous carbon chain in a branched alkane; it fixes the root word.
11. What are substituents?
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Groups attached to the parent chain that are not part of the parent chain itself.
12. How is numbering chosen when there is one substituent?
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Start from the end that gives the lowest number (locant) to the carbon carrying the substituent.
13. How is numbering chosen when there are multiple substituents?
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Choose the direction that gives the lowest sum of locants to the substituent-bearing carbons.
14. What prefixes indicate two, three and four identical substituents?
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di-, tri-, tetra- respectively, with the locant repeated each time.
15. In what order are substituent names written?
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Alphabetical order.
16. Physical state of $C_1$–$C_4$ alkanes at room temperature?
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Colourless, odourless gases.
17. Physical state of $C_5$–$C_{17}$ alkanes?
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Colourless, odourless liquids.
18. Physical state of alkanes with $C_{18}$ and above?
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Colourless, odourless solids.
19. Are alkanes soluble in water? Why?
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No — they are nonpolar and insoluble in polar solvents like water; they dissolve in benzene, ether and $CCl_4$.
20. How does boiling point change with carbon number?
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It increases, rising by about $20$–$30^\circ C$ for each added $-CH_2-$ group.
21. Why do straight-chain alkanes have higher boiling points than branched isomers?
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Straight chains have more surface contact, stronger London dispersion forces, and hence higher boiling points.
22. Why do even-carbon alkanes have higher melting points than odd-carbon neighbours?
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Even-carbon alkanes pack more symmetrically, allowing greater intermolecular attraction in the crystal lattice.
23. What are the approximate C–H and C–C bond lengths in alkanes?
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C–H $\approx 1.09\ \AA$; C–C $\approx 1.54\ \AA$.
24. Give the IUPAC name for $CH_3$–$CH_2$–$CH_2$–$CH_3$.
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Butane.
25. Name the compound: $CH_3$–$CH_2$–$CH_2$–$CH_2$–$CH_3$.
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Pentane.
26. What is the correct name for a hexane chain with methyl groups at positions 2, 3 and 5?
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2,3,5-trimethylhexane.
27. Why does 2,3,5-trimethylhexane beat 5,4,2-trimethylhexane?
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Numbering from the left gives locant sum $2+3+5=10$, lower than $5+4+2=11$.
28. What is a homologous series?
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A family of compounds with the same general formula and similar chemical properties, differing by one $-CH_2-$ unit.
29. Which reagents do alkanes resist?
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Potassium permanganate ($KMnO_4$), sodium dichromate ($Na_2Cr_2O_7$), nitric acid ($HNO_3$) and hydrogen peroxide ($H_2O_2$).
30. Are alkanes lighter or heavier than water?
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Lighter than water.
31. What is the geometry around each carbon in methane?
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Symmetrical tetrahedral with bond angles of about $109.5^\circ$.
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