Learning Objective

Identify and name an easy (straight-chain) alkanes offered formulas and also write formulas for straight-chain alkanes provided their names.

We started our research of essential chemistry in chapter 4 "Covalent Bonding and an easy Molecular Compounds" through the hydrocarbonsThe easiest organic compound, composed of carbon and hydrogen atoms only., the easiest organic compounds, which are composed of carbon and also hydrogen atoms only. As we noted, there room several different kinds that hydrocarbons. They are differentiated by the species of bonding between carbon atoms and the properties that result from that bonding. Hydrocarbons with just carbon-to-carbon solitary bonds (C–C) and existing together a continuous chain the carbon atoms additionally bonded to hydrogen atoms are called alkanes (or saturated hydrocarbons)A hydrocarbon with just carbon-to-carbon single bonds and existing together a continuous chain that carbon atoms additionally bonded to hydrogen atoms. Saturated, in this case, way that every carbon atom is bonded to 4 other atoms (hydrogen or carbon)—the most possible; there are no dual or triple bond in the molecules.

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The native saturated has the same meaning for hydrocarbons as it does for the diet fats and also oils: the molecule has no carbon-to-carbon twin bonds (C=C). (For more information about fats and also oils, check out Chapter 17 "Lipids", section 17.1 "Fatty Acids" and Section 17.2 "Fats and Oils".)

We introduced the three simplest alkanes—methane (CH4), ethane (C2H6), and also propane (C3H8)—in thing 4 "Covalent Bonding and straightforward Molecular Compounds", section 4.6 "Introduction to Organic Chemistry". They are shown again in number 12.1 "The Three easiest Alkanes". The level representations shown do no accurately portray bond angles or molecule geometry. Methane has actually a tetrahedral form that chemists regularly portray with wedges describe bonds coming out towards you and dashed present indicating bonds the go ago away indigenous you. (For much more information around the form of molecules, view Chapter 4 "Covalent Bonding and an easy Molecular Compounds", ar 4.5 "Characteristics that Molecules".) An plain solid line shows a link in the aircraft of the page.

Figure 12.1 The Three most basic Alkanes


Recall from thing 4 "Covalent Bonding and straightforward Molecular Compounds", section 4.5 "Characteristics of Molecules" that the VSEPR theory properly predicts a tetrahedral shape for the methane molecule (Figure 12.2 "The Tetrahedral Methane Molecule").

Figure 12.2 The Tetrahedral Methane Molecule


Methane (CH4), ethane (C2H6), and also propane (C3H8) space the start of a series of compound in which any type of two members in a sequence differ by one carbon atom and also two hydrogen atoms—namely, a CH2 unit. The first 10 members that this series are provided in Table 12.2 "The an initial 10 Straight-Chain Alkanes".

Table 12.2 The an initial 10 Straight-Chain Alkanes

name Molecular Formula (CnH2n + 2) Condensed structural Formula variety of Possible Isomers
methane CH4 CH4
ethane C2H6 CH3CH3
propane C3H8 CH3CH2CH3
butane C4H10 CH3CH2CH2CH3 2
pentane C5H12 CH3CH2CH2CH2CH3 3
hexane C6H14 CH3CH2CH2CH2CH2CH3 5
heptane C7H16 CH3CH2CH2CH2CH2CH2CH3 9
octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 18
nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 35
decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 75

Consider the collection in number 12.3 "Members the a Homologous Series". The sequence starts with C3H8, and also a CH2 unit is included in each step moving up the series. Any kind of family of compounds in which adjacent members different from each various other by a definite aspect (here a CH2 group) is dubbed a homologous seriesAny household of compound in which adjacent members differ from each other by a definite factor.. The members of together a series, called homologs, have actually properties that differ in a regular and also predictable manner. The principle of homology provides organization come organic chemistry in lot the same way that the periodic table gives organization to inorganic chemistry. Instead of a bewildering array of individual carbon compounds, we can study a couple of members of a homologous series and native them deduce several of the properties of other compounds in the series.

Figure 12.3 Members that a Homologous Series


Each doing well formula incorporates one carbon atom and also two hydrogen atoms more than the ahead formula.

The rule of homology allows us to write a general formula because that alkanes: CnH2n + 2. Utilizing this formula, we can write a molecular formula for any alkane v a given number of carbon atoms. For example, one alkane v eight carbon atoms has the molecule formula C8H(2 × 8) + 2 = C8H18.

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Use the basic formula because that alkanes to write the molecule formula the the alkane through 12 carbon atoms.




Key Takeaway

an easy alkanes exist together a homologous series, in which nearby members differ by a CH2 unit.