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7.3: Overview of Functional Groups Based on Atom Hybridization

  • Page ID
    215704
  • I. HYDROCARBONS

    - Substances containing only carbon and hydrogen.

    Alkanes C-C Only sp3 carbon present (technically not a functional group)
    Alkenes C=C At least one \(\pi\) -bond between two spcarbons present
    Alkynes C \(\equiv\) C At least one triple bond between two sp carbons present

     

    II. ALKYL HALIDES, OR HALOALKANES

    - Substances containing at least one C-X bond, where X=F, Cl, Br, or I.

    ALKYL FLUORIDES C-F

    ALKYL BROMIDES C-Br

    ALKYL CHLORIDES C-Cl

    ALKYL IODIDES C-I

     

    III. GROUPS CONTAINING OXYGEN

    - Both carbon and oxygen can be sp3 or sp2 hybridized, or a combination of both.

    ALCOHOLS clipboard_e16ca81718dcc19551cd32d74df0432bc.png and ETHERS clipboard_eedcb679fa63cf3133457ecb0e5c5a4f3.pngsp3 Oxygen

    ALDEHYDES clipboard_ed66dcaadf7934e13f671bf9bbbdc33aa.png and KETONES clipboard_ebe50dacd517c003ff7282b783f8c9b95.pngsp2 Oxygen

    CARBOXYLIC ACIDS clipboard_e3a72d664e34e84487fd4e497fa396873.png, ESTERS clipboard_eb0e3eac41f7813794fdfa4bfee4be191.png, and ANHYDRIDES clipboard_ea4c5ce341028987771bfaffcd72628f3.png

    These functional groups contain both sp3 and sp2 oxygen.

    ACID, OR ACYL HALIDES clipboard_e9ddd2121ea58eaca89c90080c7509960.pngThe most common halogens used are chlorine and bromine

     

    IV. GROUPS CONTAINING NITROGEN

    - They may also contain other elements. For example amides also contain oxygen

    AMINES clipboard_e60ac389367fa125c444da5f259cbd8e1.png (sp3 Nitrogen), AMIDES clipboard_e4f3501e00f67a390b4b1ea089ddedf6f.png and NITRILES clipboard_e231e7fc6b8887e43f9ca22aa58119b06.png(sp Nitrogen)

    Before proceeding, it is important to emphasize that beginning organic chemistry students must get used to seeing alkane chains from different angles, perspectives, and positions, as shown below using line-angle formulas for linear alkanes.

    Hexane clipboard_eae7eaf34d2df0d6d756e0bb8729a8404.png

    Octane clipboard_eba121f81d7c3f23806579ae26307caee.png

    BRANCHED ALKANES and ALKYL GROUPS.

    When naming branched alkanes by IUPAC rules, identify and name the longest continuous carbon chain first. Then identify the branch, or branches. The branches are called alkyl groups. For example, a one carbon branch is called a methyl group. The names of alkyl groups are the same as those of analogous alkanes, except that their names end in -yl, instead of -ane. The following are examples of the most common alkyl groups encountered in introductory organic chemistry courses. Alky groups never exist by themselves. In alkanes they are always attached to a higher priority chain and are therefore sometime called substituents. The point of attachement is indicated by a dash.

    clipboard_e96785efd8d13f8f404aca9bc7df0a71f.png

    In the following example, the longest continuous carbon chain has five carbons. Therefore the parent alkane is pentane. There is a methyl group attached to this chain. The molecule is then named methylpentane. Finally, the exact position of the methyl group is specified by numbering the main chain from the end closest to the methyl group. The complete IUPAC name for this alkane is 2-methylpentane.

    clipboard_eae80fbc3eca0286f2fcfd420b44e404c.png

    In the following examples the alkyl groups are shown in red. Students must get used to line-angle formulas as early as possible.

    clipboard_e2a3a0f51c8cfa176e2efd407fdb353b9.png

    Make sure you’ve identified the longest continuous carbon chain correctly. Otherwise you might end up with the wrong name. The longest continuous carbon chain doesn’t always have to be a horizontal row of carbons. It can take twists and turns. That’s why one must get used to visualize molecules from different angles and perspectives. The following example illustrates the correct name and an incorrect name based on a horizontal row of carbons.

    clipboard_e80eaf00fdcdd507cddb21f0e4fb59d51.png

    The correct name above illustrates what’s done when there are several substituents of the same kind. First identify their positions, then use the prefixes di-, tri-, tetra-, etc. to indicate how many are present.

    clipboard_e34a78593ef1df9ee46c75b02fa117fea.png

    If there are substituents of different kinds present, name them in alphabetical order (e.g. ethyl before methyl). Prefixes such as di-, tri-, tetra-, etc. are ignored when alphabetizing.

    clipboard_e11ae1f7a23d33e10ffa6f49ec56ced51.png

    If there are several options for choosing the longest continuous carbon chain, choose the one that yields the simplest name. That means:

    (a) The longest continuous carbon chain has the greatest number of substituents, and

    (b) The name has the lowest set of numbers indicating the substituent positions.

    clipboard_eb037969ce91e051c3ea9fbc815ecb9ca.png

    Complex alky groups (those which are branched themselves) are named as if they were alkanes, but the name ends in -yl and is enclosed in parenthesis. The carbon from which the substituent atttaches to the main chain is automatically number 1.

    clipboard_e633267930771425ed6a8f2368fe1163d.png

    Although common alkyl group names such as isopropyl and t-butyl are commonly used as part of IUPAC names, strict IUPAC rules call for naming them as complex substituents. Common names are used because they are easier to say, shorter, and save paper and ink in scientific publications when they have to be used repeatedly in manuscripts.

    clipboard_e292c8e71f272a540bb09350044462f41.png

    ALKANES: COMMON NAMES

    Like alkyl groups, alkanes can also have common names. Common, or “trivial” names are rarely used for straight chain alkanes, but are frequently used for branched alkanes. The following pages contain some terminology that organic chemistry students must memorize or become familiar with, due to the fact that it is widely used in textbooks and in the chemical literature.

    ISOALKANES and ISOALKYL GROUPS

    The iso structural unit consists of two methyl groups attached to a common carbon. When this unit is present in an alkane or alky group, the common name starts with the prefix iso.

    clipboard_e3a7a9548aecfcc1837d51325e9f5cda7.png

    When this unit is present in alkanes, the molecule is named according to the total number of carbon atoms present, but the name starts with the prefix iso. The smallest possible isoalkane is then isobutane.

    clipboard_e708d703d79100a874c50f93fcd5eb02c.png

    The same rules apply to isoalkyl groups, but their names end in -yl. The smallest possible isoalky group is the isopropyl group, because alkyl groups are always attached to another carbon chain.

    clipboard_e9d2987787784908b44e36517245d555c.png

    PRIMARY, SECONDARY, AND TERTIARY CARBONS.

    Another bit of terminology associated with common names refers to the connectivity of sp3 carbons in alkanes and alkyl groups. A primary carbon is one that is covalently attached to only one other carbon. A secondary carbon is one attached to two other carbons. A tertiary carbon is attached to three other carbons. This definition implies that methane cannot have any such carbons, since it consists of only one carbon atom. Likewise, this terminology applies only to sp3 carbons. Other types are not defined in this way. In the following examples the carbons in question are indicated as dots.

    clipboard_ebc5e3cacf565953784cb7e65b495b557.png

    The carbon atoms directly engaged π-bonding in alkenes are referred to as vinylic. An sp3 carbon directly attached to a vinylic carbon is referred to as allylic. Finally, an sp3 carbon directly attached to a benzene ring is referred to as benzylic.

    clipboard_e2b7a4c558e342ef7a46dcc2f80ede397.png

    Although it is emphasized that this is terminology used in common names, it is widely used and therefore important. It constitutes the basis for characterizing not only alkanes, but also other types of compounds where the main functionality is attached to a carbon that belongs to one of the types described. Thus we can have primary alcohols, secondary chlorides, allylic hydrogens, etc. based on the type of carbon to which they are attached.

    clipboard_e22ee699fed54a0b25d8af9205c16a2db.png

    This terminology is also used to identify the point of attachment of certain alkyl groups to the main chain of a molecule, based on the type of carbon from which the alkyl group connects to the main chain. For example four different butyl groups are possible, depending on their structure and their point of attachemnt to the main carbon chain. Two of them are named sec-butyl and tert-butyl (or t-butyl) because they connect to the main chain from a secondary and a tertiary carbon respectively.

    clipboard_e660877f102d4bb39439ee36c368434fc.png

    The square represents the main carbon chain of a molecule

    While there are four possible butyl groups, only two possible butanes exist. They are n-butane and isobutane (once again, these are common names). Likewise, there are three possible pentanes called n-pentane, isopentane, and neopentane.

    clipboard_e611e3556f1305098c719395c887ce664.png

    The nomenclature concepts presented in the previous sections also apply to cycloalkanes. Please consult your organic chemistry textbook for examples and additional practice exercises.

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