How To Draw Isomers Of Alkanes
3.2: Alkanes and Alkane Isomers
- Folio ID
- 31399
Later on completing this section, yous should be able to
- draw the Kekulé structure, condensed construction and autograph structure of each of the get-go x straight-concatenation alkanes.
- name each of the first ten straight-chain alkanes, given its molecular formula, Kekulé structure, condensed structure or autograph structure.
- explain the difference in structure between a straight- and a branched-chain alkane, and illustrate the difference using a suitable example.
- explain why the number of possible isomers for a given molecular formula increases as the number of carbon atoms increases.
- draw all the possible isomers that correspond to a given molecular formula of the blazon Cn H2n+2, where northward is ≤ 7.
Make sure that yous tin ascertain, and use in context, the key terms below.
- branched-chain paraffin
- constitutional or structural isomer
- homologous series
- isomer
- saturated hydrocarbon
- directly-chain alkane (or normal alkane)
A series of compounds in which successive members differ from one some other by a CH2 unit is chosen a homologous series. Thus, the series CHiv, CiiHsix, C3Height . . . CnH2n+2, is an example of a homologous series.
Information technology is important that you commit to memory the names of the start ten straight-concatenation alkanes (i.e., from CH4 to C10H22). You will use these names repeatedly when you begin to learn how to derive the systematic names of a large variety of organic compounds. You need not recollect the number of isomers possible for alkanes containing more than than 7 carbon atoms. Such information is available in reference books when it is needed. When drawing isomers, be conscientious not to deceive yourself into thinking that yous can draw more isomers than you lot are supposed to be able to. Remember that it is possible to depict each isomer in several different ways and y'all may inadvertently count the same isomer more once.
Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. Alkanes are often called saturated hydrocarbons considering they have the maximum possible number of hydrogens per carbon. In Section 1.7, thealkane molecule, ethane, was shown to contain a C-C sigma bail. By calculation more C-C sigma bond larger and more than complexed alkanes can be formed. Methane (CH4), ethane (CiiHvi), and propane (C3Hviii) are the beginning of a series of compounds in which any 2 members in a sequence differ by 1 carbon atom and two hydrogen atoms—namely, a CH2 unit of measurement. Any family of compounds in which adjacent members differ from each other by a definite factor (hither a CH2 group) is called a homologous series. The members of such a series, chosen homologs, take properties that vary in a regular and predictable fashion.
Methyl hydride (CH4), ethane (C2Hhalf-dozen), and propane (C3Height) are the beginning of a series of compounds in which any two members in a sequence differ past one carbon atom and two hydrogen atoms—namely, a CH2 unit. Consider the serial in Figure 25.3.iii. The sequence starts with C3H8, and a CH2 unit of measurement is added in each step moving up the series. Any family unit of compounds in which adjacent members differ from each other by a definite factor (here a CHii group) is called a homologous series. The members of such a series, called homologs, have backdrop that vary in a regular and predictable manner.
Figure 25.iii.2: Members of a Homologous Series. Each succeeding formula incorporates one carbon atom and ii hydrogen atoms more than the previous formula.
The homologous serial allows us to write a full general formula for alkanes: C north Hii due north + 2. Using this formula, we can write a molecular formula for whatsoever methane series with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C8H(ii × 8) + 2 = C8H18.
Molecular Formulas
Alkanes are the simplest family unit of hydrocarbons - compounds containing carbon and hydrogen but. Alkanes only comprise carbon-hydrogen bonds and carbon-carbon unmarried bonds. The kickoff vi alkanes are as follows:
Table \(\PageIndex{1}\): Molecular formulas for minor alkanes
| methane | CH4 |
| ethane | C2H6 |
| propane | C3Hviii |
| butane | C4H10 |
| pentane | C5H12 |
| hexane | C6H14 |
Y'all can piece of work out the formula of any of the alkanes using the general formula Cdue northH2n+2
Isomerism
All of the alkanes containing 4 or more carbon atoms show structural isomerism, meaning that there are 2 or more dissimilar structural formulas that you can draw for each molecular formula. Isomers (from the Greek isos + meros, meaning "made of the same parts") are molecules that have the same molecular formula, but have a different organization of the atoms in space. Alkanes with 1-3 carbons, methane (CH4), ethane (C2H6), and propane (CiiiH8,) do not be in isomeric forms because at that place is only one manner to suit the atoms in each formula so that each carbon atom has four bonds. Even so, C4H10, has more than possible construction. The 4 carbons can be drawn in a row to course butane or the tin can branch to form isobutane. The two compounds have different properties—for case, butane boils at −0.5°C, while isobutane boils at −eleven.7°C.
Also the molecular formula: CvH12 has three possible isomer. The compound at the far left is pentane because it has all five carbon atoms in a continuous chain. The compound in the middle is isopentane; like isobutane, it has a one CH3 branch off the 2d carbon cantlet of the continuous concatenation. The compound at the far right, discovered after the other two, was named neopentane (from the Greek neos, meaning "new"). Although all three take the same molecular formula, they accept dissimilar backdrop, including humid points: pentane, 36.1°C; isopentane, 27.vii°C; and neopentane, nine.5°C.
Of the structures testify above, butane and pentane are called normal alkanes or straight-chain alkanes, indicating that all incorporate a unmarried continuous chain of carbon atoms and tin be represented by a projection formula whose carbon atoms are in a straight line. The other structures, isobutane, isopentane, and neopentane are called called branched-chain alkanes. As the number of carbons in an akane increases the number of possible isomers too increases as shown in the table beneath.
Table \(\PageIndex{2}\): Number of isomers for hydrocarbons
| Molecular Formula | Number of Structural Isomers |
|---|---|
| CH4 | 1 |
| C2Hhalf dozen | 1 |
| CiiiH8 | one |
| C4H10 | 2 |
| C5H12 | iii |
| C6Hfourteen | 5 |
| C7Hsixteen | 9 |
| C8Hxviii | 18 |
| C9H20 | 35 |
| CtenH22 | 75 |
| CfourteenHthirty | 1858 |
| CeighteenH38 | 60,523 |
| C30H62 | 4,111,846,763 |
Akanes tin be represented in many different ways. The figure below shows some of the different ways directly-chain butane tin can be represented. Near often chemists refer to butane past the condensed structure CH3CH2CH2CHthree or n-C4H10 where n denotes a normal directly alkane series.
Note that many of these structures only imply bonding connections and do non bespeak any particular geometry. The lesser ii structures, referred to as "ball and stick" and "infinite filling" do show 3D geometry for butane. Because the four-carbon chain in butane may exist bent in various means the groups can rotate freely almost the C–C bonds. Still, this rotation does not change the identity of the compound. It is of import to realize that bending a chain does non change the identity of the chemical compound; all of the following represent the same compound, butane:
The nomenclature of straight alkanes is based on the number of carbon atoms they comprise. The number of carbons are indicated by a prefix and the suffix -1 is added to indicate the molecules is an alkane series. The prefix for three carbons is prop and then adding -i, the IUPAC name for CthreeHviii is propane. Likewise, the prefix for six is hex so the name for the straight concatenation isomer of C6H14 is called hexane. The first ten prefixes should be memorized, because these alkane names from the basis for naming many other organic compounds.
| Molecular Formula | Prefix | Condensed Structural Formula | Name |
|---|---|---|---|
| CH 4 | Meth | CH 4 | methane |
| C 2 H 6 | Eth | CH 3 CH three | ethane |
| C iii H eight | Prop | CH 3 CH 2 CH 3 | propane |
| C 4 H 10 | But | CH iii CH 2 CH 2 CH 3 | butane |
| C 5 H 12 | Pent | CH 3 CH ii CH 2 CH two CH 3 | pentane |
| C 6 H 14 | Hex | CH iii (CH two ) four CH iii | hexane |
| C seven H 16 | Hept | CH 3 (CH ii ) 5 CH 3 | heptane |
| C viii H xviii | Oct | CH 3 (CH two ) 6 CH iii | octane |
| C nine H 20 | Non | CH 3 (CH 2 ) 7 CH 3 | nonane |
| C 10 H 22 | Dec | CH 3 (CH 2 ) 8 CH 3 | decane |
Pentane, C5H12, has three concatenation isomers. If you think you can observe any others, they are merely twisted versions of the ones below. If in doubt make some models.
Exercises
Draw all of the isomers for CviH14O that comprise a half dozen carbon concatenation and an booze (-OH) functional group.
- Answer
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Describe all possible isomers for C6H14 (There are five full).
- Answer
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The superlative structure is when information technology is a 6 carbon chain. The middle row contains the 5 carbon chained isomers with branching at the twond and 3rd carbon. The bottom row contains the two 4 carbon chain isomers that can be drawn.
Depict all possible isomers for C3H8.
- Answer
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The offset structure is when an booze comes off the offset carbon. The second structure is when the alcohol is coming off the central carbon. The third structure is the merely possible ether form of C3Hviii.
Depict all possible isomers for C4HviiiOtwo that contain a carboxylic acid.
- Answer
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There are only ii possibilities.
Draw all possible isomers for CthreeH9N and bespeak whether each amine is primary, secondary, or 3rd.
- Answer
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The commencement and second structures are primary amines. The tertiary structure is a secondary amine. The last structure is a tertiary amine.
Indicate whether each of the following sets are constitutional isomers, the aforementioned chemical compound, or different compounds.
- Respond
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a) Both structures have formulas of CxH22 and have different connectivity which makes these constitutional isomers.
b) Both structures have formulas of CsevenHxvi and have the same connectivity which makes these the aforementioned compound.
c) Both structure have formulas of C7Hsixteen and have different connectivity which makes these ramble isomers.
d) The structure on the left has a formula of C9Hxx and the structure on the right has a formula of C10H22 so these are different compounds.
Draw the 5 constitutional isomers of C7Hsixteen (of the 9 total isomers possible) that take 5 carbons as the longest carbon concatenation length.
- Answer
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The 5 constitutional isomers with a v carbon chain length are shown in a higher place. Since there needs to be 7 carbons total, the 2 extra carbons are added as substituents. From left to right, the methyl group substitution blueprint is 2,2, 2,3, 2,4, and 3,3, and the last one (on right) has a iii-ethyl substituent.
The other 4 possible constitutional isomers (with different length carbon bondage) are shown below.
Source: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_%28McMurry%29/03:_Organic_Compounds-_Alkanes_and_Their_Stereochemistry/3.02:_Alkanes_and_Alkane_Isomers
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