The International Union of Pure and Applied Chemistry (IUPAC) defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula CnH2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denote any saturated hydrocarbon, including those that are either monocyclic (i.e. the cycloalkanes) or polycyclic,[2] despite their having a distinct general formula (i.e. cycloalkanes are CnH2n).
alkane alkene alkyne formula pdf 13
Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. The above list only includes differences of connectivity, not stereochemistry. In addition to the alkane isomers, the chain of carbon atoms may form one or more rings. Such compounds are called cycloalkanes, and are also excluded from the above list because changing the number of rings changes the molecular formula. For example, cyclobutane and methylcyclopropane are isomers of each other (C4H8), but are not isomers of butane (C4H10).
Branched-chain alkanes are called isoparaffins. "Paraffin" is a general term and often does not distinguish between pure compounds and mixtures of isomers, i.e., compounds of the same chemical formula, e.g., pentane and isopentane.
However, at times it may be desirable to make a section of a molecule into an alkane-like functionality (alkyl group) using the above or similar methods. For example, an ethyl group is an alkyl group; when this is attached to a hydroxy group, it gives ethanol, which is not an alkane. To do so, the best-known methods are hydrogenation of alkenes:
In the Wurtz reaction, a haloalkane is treated with sodium in dry ether to yield an alkane having double the number of carbon atoms. This reaction proceeds through a free radical intermediate and has the possibility of alkene formation in case of tertiary haloalkanes and vicinal dihalides.
Thus, they have formulas that can be drawn as cyclic alkenes, making them unsaturated. However, due to the cyclic structure, the properties of aromatic rings are generally quite different, and they do not behave as typical alkenes. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene).
In an alkene, the double bond is shared by the two carbon atoms and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious, ie the condensed formula for ethene is CH2CH2. The double or triple bond nature of a molecule is even more difficult to discern from the molecular formulas. Note that the molecular formula for ethene is C2H4, whereas that for ethyne is C2H2. Thus, until you become more familiar the language of organic chemistry, it is often most useful to draw out line or partially-condensed structures, as shown below:
The fixed and rigid nature of the double bond creates the possibility of an additional chiral center, and thus, the potential for stereoisomers. New stereoisomers form if each of the carbons involved in the double bond has two different atoms or groups attached to it. For example, look at the two chlorinated hydrocarbons in Figure 8.8. In the upper figure, the halogenated alkane is shown. Rotation around this carbon-carbon bond is possible and does not result in different isomer conformations. In the lower diagram, the halogenated alkene has restricted rotation around the double bond. Note also that each carbon involved in the double bond is also attached to two different atoms (a hydrogen and a chlorine). Thus, this molecules can form two stereoisomers: one that has the two chlorine atoms on the same side of the double bond, and the other where the chlorines reside on opposite sides of the double bond.
As we saw in Chapter 7, small alkanes can be formed by the process of thermal cracking. This process also produces alkenes and alkynes. In comparison to alkanes, alkenes and alkynes are much more reactive. In fact, alkenes serve as the starting point for the synthesis of many drugs, explosives, paints, plastics and pesticides. Alkanes can undergo five major types of reactions: (1) Combustion Reactions, (2) Addition Reactions, (3) Elimination Reactions, (4) Substitution Reactions, and (5) Rearrangement Reactions. Since combustion reactions were covered heavily in Chapter 7, and combustion reactions with alkenes are not significantly different than combustion reactions with alkanes, this section will focus on the later four reaction types.
Most reactions that occur with alkenes are addition reactions. As the name implies, during an addition reaction a compound is added to the molecule across the double bond. The result is loss of the double bond (or alkene structure), and the formation of the alkane structure. The reaction mechanism of a reaction describes how the electrons move between molecules to create the chemical reaction. Note that in reaction mechanism diagrams, as shown in Figure 8.15, curved arrows are used to show where electrons are moving. The reaction mechanism for a generic alkene addition equation using the molecule X-Y is shown below:
Thus, the overall structure is very stable compared to other alkenes and benzene rings do not readily undergo addition reactions. They behave more similarly to alkane structure and lack chemical reactivity. One of the few types of reactions that a benzene ring will undergo is a substitution reaction. Recall from Chapter 7 that in substitution reactions an atom or group of atoms is replaced by another atom or group of atoms. Halogenation is a common substitution reaction that occurs with benzene ring structures. In the diagram below, notice that the hydgrogen atom is substituted by one of the bromine atoms.
Alkanes, alkenes and alkynes are simple hydrocarbon chains with no functional groups. The simplest organic compounds are the alkanes. Alkanes have only single bonds between carbon atoms and are called saturated hydrocarbons. Alkenes have at least one carbon-carbon double bond. Alkynes have one or more carbon-carbon triple bonds. Alkenes and alkynes are called as unsaturated hydrocarbons. Alkanes have the general formula of CnH2n+2 where n is the number of carbon atoms. Alkenes have the general formula CnH2n. The general formula for alkynes is CnH2n-2. Acetylene is the simplest alkyne with the formula as C2H2.
Alkanes are non-polar compounds and insoluble in water. They have low boiling and melting points. In general alkanes & cycloalkanes have low reactivity & are used as solvents in organic chemistry. Natural gas, camping gas, lighter gas and much of gasoline are all alkanes. All the alkanes burn but they need a lot of air or oxygen to burn completely. Alkenes and alkynes are more reactive than alkanes. Unsaturated compounds (with double or triple bonds) form addition & cycloaddition compounds, where two substances combine to form a single substance.
The simplest alkane is methane (CH4), a colorless, odorless gas that is the major component of natural gas. In larger alkanes whose carbon atoms are joined in an unbranched chain (straight-chain alkanes), each carbon atom is bonded to at most two other carbon atoms. The structures of two simple alkanes are shown in Figure \(\PageIndex1\), and the names and condensed structural formulas for the first 10 straight-chain alkanes are in Table \(\PageIndex1\). The names of all alkanes end in -ane, and their boiling points increase as the number of carbon atoms increases.
The simplest alkenes are ethylene, C2H4 or CH2=CH2, and propylene, C3H6 or CH3CH=CH2 (part (a) in Figure \(\PageIndex2\)). The names of alkenes that have more than three carbon atoms use the same stems as the names of the alkanes (Table \(\PageIndex1\) "The First 10 Straight-Chain Alkanes") but end in -ene instead of -ane.
Just as a number indicates the positions of branches in an alkane, the number in the name of an alkene specifies the position of the first carbon atom of the double bond. The name is based on the lowest possible number starting from either end of the carbon chain, so CH3CH2CH=CH2 is called 1-butene, not 3-butene. Note that CH2=CHCH2CH3 and CH3CH2CH=CH2 are different ways of writing the same molecule (1-butene) in two different orientations.
In a cyclic hydrocarbon, the ends of a hydrocarbon chain are connected to form a ring of covalently bonded carbon atoms. Cyclic hydrocarbons are named by attaching the prefix cyclo- to the name of the alkane, the alkene, or the alkyne. The simplest cyclic alkanes are cyclopropane (C3H6) a flammable gas that is also a powerful anesthetic, and cyclobutane (C4H8) (part (c) in Figure \(\PageIndex2\)). The most common way to draw the structures of cyclic alkanes is to sketch a polygon with the same number of vertices as there are carbon atoms in the ring; each vertex represents a CH2 unit. The structures of the cycloalkanes that contain three to six carbon atoms are shown schematically in Figure \(\PageIndex3\).
The previous post in the series on alkynes was entitled, Alkynes Are A Blank Canvas. Alkynes are a blank canvas because on top of their own transformations, through partial reduction (Na/NH3 or Lindlar) alkynes can also be transformed to alkenes, (which themselves have a host of reactions) or even alkanes (which can then be transformed to alkyl halides, which also have a host of reactions).
Sir, my professor told me that you can dehydrogenate an alkane to an alkene with the use of a metal catalyst like Pt. Also, I read in a book that you can reduce an alkyl halide to an alkane with LiAlH4. Why are these not in your map?
Like alkanes, alkenes can be straight chain or branched chain. The double bond between the carbon atoms can be anywhere in the chain. It is also possible to have cyclic alkenes (with or without branches off the ring). Note: benzene is sometimes drawn as a hexagon with three alternating double bonds, making it look like an cyclic alkene. IT IS NOT AN ALKENE. 2ff7e9595c
Comments