Understanding Alkanes and Cycloalkanes: Structure, Nomenclature, and Properties, Study notes of Organic Chemistry

Download Understanding Alkanes and Cycloalkanes: Structure, Nomenclature, and Properties and more Study notes Organic Chemistry in PDF only on Docsity! Alkanes 2.1 What are Alkanes and Cycloalkanes? Hydrocarbons are compounds that is composed of hydrogen ("hydro-") and carbon atoms. There are two kinds of hydrocarbon based on the type of bonds it has. These are saturated and unsaturated hydrocarbons. Saturated hydrocarbons are composed of all single bonds. While, unsaturated hydrocarbons are composed of compounds with multiple bonds (e.g. double bonds, triple bonds and benzene rings). Looking at the dictionary, the word saturated means "filled" and saturated hydrocarbons are "filled" with the maximum number of hydrogen atoms the carbon atom can carry (which is 4 by the way). Look at the example below and compare the number of hydrogen atoms ethane, ethene and ethyne can have. Remember that your carbon atom can only carry 4 bonds. See that each time you increase the number of bonds between the carbon atoms, two hydrogen atoms are lost. Thus, ethane is considered to be an example of a saturated hydrocarbon. Ethene (an alkene) and ethyne (alkyne) are examples of unsaturated hydrocarbon. Alkanes are also referred to as aliphatic hydrocarbons because the high molecular weight alkanes have physical properties that resemble what we find in animal fats and plant oils. Alkanes can be straight chain (acyclic) and cyclic (cycloalkanes). Notice the difference between molecular formula the n-hexane (C6H14) and cyclohexane (C6H12). As we created additional bond to create a cyclic structure, two hydrogen atoms are lost in the cyclohexane molecule. Thus, the general formula for straight chain alkane is CnH2n+2 while for cycloalkanes is CnH2n. Another difference between the straight chain and cyclic structures is their ability to rotate. The bonds between carbons in a straight chain alkane can freely rotate while the ones in the cyclic form cannot. Forcing the carbon-carbon bond to rotate in a cyclic structure will break it structure. With this you expect that the straight chain alkane will have infinite number of 3-D shape or conformations. But they prefer the least crowded conformations as we saw in the video. Chemist represents these different conformations as Newman projections. Newman projections view carbon- carbon bond directly end-on and represent the two carbon atoms as circles (Front and back carbon). See the representation of butane below. There are mainly two kinds of Newman projection for butane. These are staggered and eclipsed. Staggered conformations are the lowest-energy and most stable conformation. They are lowest in energy because there is less steric hindrance from the two methyl groups. Unlike the eclipse conformation, you can see that the two methyl groups are hovering each other, thus there's steric hindrance happening. Staggered conformations happen when the bonds rotate as far apart from each other. There are two kinds of staggered conformations: the gauche (B) and anti (C). Look at the difference between B and C Newman projection, gauche happens when the bonds rotate 60° and the two methyl are "somewhat" far from each other. Unlike the anti-staggered conformation, the two methyl's are 180° away from each other. This is the most stable conformation. As for cycloalkanes, we will limit our discussion to cyclopentane and cyclohexane. Cyclopentane's most stable conformation is the envelope conformation, where in the four carbon atoms are on the plane and the fifth carbon is bent out of the plane. It is just like an envelope with its flap bent upward. Cyclohexane's most stable conformation is the chair conformation. The 12 C-H bonds are arranged in two different orientations. Six of them are in axial positions and another six are in equatorial positions. The rules of the IUPAC system for naming alkanes are as follows: 1. ) Identify the parent chain. See figure below, Figure 2.4.3. Identifying parent chains 2. ) Number the carbon atoms in the parent chain from the end of the chain nearest a substituent. The substituent should have the lowest possible number. Figure 2.4.4. How to properly number parent chains of a singly branched alkane (2). 3. ) If only one alkyl group is present, name and locate it (by number). Next name the parent carbon chain. Name is written as one word with a hyphen between the number (location) and the name of the alkyl group. Figure 2.4.5. how to properly name a singly branched alkane (2). 4. ) If two or more of the same kind of alkyl group are present in a molecule indicate the number with Greek numerical prefixes (di-, tri-, tetra-, penta-, etc). In addition, a number specifying the location of each identical group must be included. These position numbers, separated by commas, precede the number prefix. Numbers are separated from words by hyphen (see figure 2.4.6). Figure 2.4.6. how to properly name branched alkane with similar substituents (2). 5. ) When two kinds of alkyl groups are present on the same carbon chain, number each group separately and list the names of the alkyl groups in alphabetical order (see example in figure 2.4.7 a and b). However, Greek prefixes such as di-, tri-, sec- and tert- are left out when you arrange substituents according to alphabetical order (see example in figure 2.4.7 c). Figure 2.4.7. how to properly name branched alkane with different substituents (1, 2). Always remember to follow IUPAC punctuation rules: a.) Separate numbers from each other by commas b.) Separate number from letters by hyphens c.) Do not add a hyphen or a space between the last-named substituent and the parent alkane. d.) Arrange substituents in alphabetical order but leave out di-, tri-, sec- and tert-. Cycloalkanes are named similarly to straight chain alkanes. Cycloalkanes have a prefix of cyclo- to refer that they are cyclic and not straight chain (see examples in figure 2.4.8). Figure 2.4.8. how to properly name branched alkane with different substituents (2). For cycloalkanes with substituents, the following are the IUPAC rules to follow: a.) For singly substituted cycloalkanes parent chain, no need locate the substituent by number (see figure 2.4.9-A). b.) For di- substituted cycloalkane parent chain, the carbon atoms in the ring are numbered beginning with the substituent of higher alphabetical priority and proceeding in the direction (counter clockwise or clockwise) that gives the other substituent the lower number (see figure 2.4.9-B). c.) For three or more ring substituent are present, ring numbering begins at the substituent that leads to the lowest set of location numbers. When two or more equivalent numbering set exist, alphabetical priority among substituents determine the set used (see figure 2.4.9-C). Figure 2.4.9. how to properly name branched cycloalkanes (2). d.) When ring system contains fewer carbon atoms than an alkyl group attached to it, t he alkyl group is named as the parent chain (see figure 2.4.10). Figure 2.4.10. how to properly name alkanes with cycloalkanes as substituent (2). e.) Specify cis- and trans- isomerism of cycloalkanes as prefixes (see figure 2.4.11). Figure 2.4.11. Specifying cis and trans isomerism for cycloalkanes (2).