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Names of alkyl groups

Names of alkyl groups. Alkyl groups are named by replacing the suffix –ane with –yl. Therefore:. Propane alkyl groups. Propane has two kinds of hydrogens: 6 primary and two primary. Removal of any of the primary carbons leads to the formation of propyl or n-propyl group

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Names of alkyl groups

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  1. Names of alkyl groups • Alkyl groups are named by replacing the suffix –ane with –yl. Therefore:

  2. Propane alkyl groups • Propane has two kinds of hydrogens: 6 primary and two primary. • Removal of any of the primary carbons leads to the formation of propyl or n-propyl group • Removal of any of the secondary hydrogens leads to the formation of the isopropyl group.

  3. The butyl isomers • Removal of different hydrogens from the two butanes leads to the formation of four butyl groups- n-butyl, sec-butyl-, isobutyl, and tert-butyl- • As an exercise write the structures of the above.

  4. How do we name this particular structure? Methylethylisopropylnonane This name is still ambiguous as it does not tell at which position each of the alkyl groups are placed Use IUPAC RULES to overcome ambiquity

  5. IUPAC name of the nonane • 4-Ethyl-2-methyl-6-isopropylnonane • Give the structure of the compound 3,5-diethyl-3,4-dimethyloctane.

  6. Structure of 3,5-diethyl-3,4-dimethyloctane. • Write the structural formula of the compound whose IUPAC name is 2, 2, 6- trimethyl-3- ethylheptane.

  7. What is the name of the given structures?

  8. IUPAC rules of naming of alkanes

  9. IUPAC rules ………. • If two or more substituents are the same, the prefixes “di,” “tri,” and “tetra” are used to indicate how many identical substituents the compound has. The numbers indicating the locations of the identical substituents are listed together, separated by commas.

  10. IUPAC Rules………. • The prefixes di, tri, tetra, sec, and tert are ignored in alphabetizing substituent groups, but the prefixes iso, neo, and cyclo are not ignored.

  11. IUPAC Rules………. • When both directions lead to the same lowest number for one of the substituents, the direction is chosen that gives the lowest possible number to one of the remaining substituents.

  12. IUPAC Rules……… • If the same substituent numbers are obtained in both directions, the first group cited receives the lower number.

  13. IUPAC Rules………… • If a compound has two or more chains of the same length, the parent hydrocarbon is the chain with the greatest number of substituents.

  14. Names of common substituents • -F fluoro, -CN cyano • -Br bromo -NO2nitro • -Cl chloro -NH2 amino • -I iodo • The following compound is named 3-bromo-2-chloro-4 methylpentane

  15. Complex Substituents • If the branch has a branch, number the carbons from the point of attachment. • Name the branch off the branch using a locator number. • Parentheses are used around the complex branch name. 1-methyl-3-(1,2-dimethylpropyl)cyclohexane

  16. Tutorial 1 Write the systematic (IUPAC) name for the following compounds

  17. Physical properties of alkanes • Alkanes experience inter-molecular van der Waals forces. Stronger inter-molecular van der Waals forces give rise to greater boiling points of alkanes. • There are two determinants for the strength of the van der Waals forces: • the number of electrons surrounding the molecule, which increases with the alkane's molecular weight • the surface area of the molecule

  18. Physical properties of Alkanes…….. • Under standard conditions:- • CH4 to C4H10 alkanes are gaseous • C5H12 to C17H36 they are liquids • After C18H38 they are solids.

  19. Physical properties of Alkanes • Alkanes are not very reactive and have little biological activity. • Alkanes can be viewed as a molecular scaffold upon which can be hung the interesting biologically-active/reactive portions (functional groups) of the molecule. • Alkanesdo not conduct electricity, nor are they substantially polarized by an electric field. For this reason they do not form hydrogen bonds and are insoluble in polar solvents such as water.

  20. Properties of Alkanes (Solubility) • Alkanes are said to be hydrophobic in that they repel water. • Remember that like dissolves like and hence alkanes are water hating. • Their solubility in nonpolar solvents is relatively good, a property that is called lipophilicity. • Different alkanes are, for example, miscible in all proportions among themselves.

  21. Properties of Alkanes…. (Density) • The density of the alkanes usually increases with increasing number of carbon atoms, but remains less than that of water(their density is less than 1 g/mL). • Hence, alkanes form the upper layer in an alkane-water mixture

  22. Physical properties • As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has almost a linear relationship with the size (molecular weight) of the molecule. • As a rule of thumb, the boiling point rises 20 - 30 °C for each carbon added to the chain; this rule applies to other homologous series

  23. Properties of alkanes….(BPts.)

  24. Properties of Alkanes… • A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules. • For example, compare isobutane and n-butane, which boil at -12 and 0 °C respectively.

  25. Variation of BPts. • The decrease in boiling point with increased branching can be explained as follows: as branching increases, the molecule becomes more compact, decreasing its surface area. • With a smaller surface area, the intermolecular forces of attraction are diminished, requiring less energy to go from the liquid to gaseous state.

  26. Properties of Alkanes…(BPts.) • 2,2-dimethylbutane and 2,3-dimethylbutane boil at 50 and 58 °C, respectively.

  27. Properties of Alkanes…(BPts.) • For the latter case, two molecules 2,3-dimethylbutane can "lock" into each other better than the cross-shaped 2,2-dimethylbutane, hence the greater van der Waals forces, resulting in higher boiling point.

  28. SAQ • 9.3 Arrange the following compounds in order of increasing boiling points: n-hexane; 2,2-dimethylbutane; 2-methylpentane.

  29. Solution to SAQ • The order is : • 2,2-Dimethylbutane< 2-methylpentane< hexane

  30. Properties of Alkanes….(MPts.)

  31. Properties of alkanes……. (MPt.) • The melting points also increase with increasing molecular weight but less regularly compared to boiling points. • Melting points increase with increasing carbons (less for odd-number of carbons). • Alkanes with even number of carbons are more symmetrical than those with odd number of carbon atoms. • The even-numbered alkanes thus pack more closely in the crystalline state and hence have greater attractive forces, higher melting points.

  32. Branching increases melting points. Branched alkanes pack more efficiently into a crystalline structure, so have higher m.pts. Highly Symmetrical structures have abnormally high melting points. The example below is a clear illustration of this. Properties of alkanes…. (MPts.)

  33. Major Uses of Alkanes • C1-C2: gases (natural gas) • C3-C4: liquified petroleum (LPG) • C5-C8: gasoline • C9-C16: diesel, kerosene, jet fuel • C17-up: lubricating oils, heating oil • Origin: petroleum refining

  34. Hazards of Alkanes • Methane is explosive when mixed with air (1 – 8% CH4) and is a strong greenhouse gas: Other lower alkanes can also form explosive mixtures with air. The lighter liquid alkanes are highly flammable, although this risk decreases with the length of the carbon chain. • Pentane, hexane, heptane, and octane are classed as dangerous for the environment and harmful. • The straight-chain isomer of hexane is a neurotoxin.

  35. These are types of alkanes which have one or more rings of carbon atoms in the chemical structure of their molecules. Formula: CnH2n They are nonpolar and insoluble in water They have a compact shape Cycloalkanes

  36. Cycloalkanes • Cycloalkanes are classified into small, common, medium, and large cycloalkanes, where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane, cycloheptane are the common ones, cyclooctane through cyclotridecane are the medium ones, and the rest are the larger ones

  37. Properties of CycloAlkanes • Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points, melting points, and densities than alkanes. This is due to stronger London forces because the ring shape allows for a larger area of contact. • Cycloalkanesexhibit almost the same degree of unreactivity as alkanes, due to their containing only unreactive C-C and C-H bonds; however, the ring strain can cause cycloalkanes to be more reactive.

  38. Nomenclature of Cycloalkanes

  39. The cycloalkane is usually the base compound Number carbons in ring if more than one substituent are present. First in alphabet gets lowest number. You may have cycloalkyl attachments to a chain. => Naming substituted cycloalkanes

  40. Cis: like groups on same side of ring Trans: like groups on opposite sides of ring Cis-Trans Isomerism

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