# shape and structure of organic compounds

In our study of organic chemistry, it will become extremely important to be able to quickly recognize the most common functional groups, because they are the key structural elements that define how organic molecules react. The word organic has different meanings. The name is therefore 2-methylpentane. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. theless, such "space-filling" models made with truncated balls held together with snap fasteners are used widely to determine the possible closeness of approach of groups to each other and the degree of crowding of atoms in various arrangements (see Figure 2-3). For example, the CH3 group derived from methane (CH4) results from subtracting one hydrogen atom and is called a methyl group. It is not easy to visualize the possible arrangements of the bonds in space and it is very helpful to have some kind of mechanical model that reflects the molecular geometry, including at least an approximation to the relative lengths of the bonds. Organic fertilizer, such as cow manure, is organic in the original sense; it is derived from living organisms. You may need to download version 2.0 now from the Chrome Web Store. 2-1 STRUCTURAL FORMULAS The building block of structural organic chemistry is the tetravalent carbon atom. When these rules are followed, every unique compound receives its own exclusive name. We begin our study of organic chemistry with the hydrocarbons, the simplest organic compounds, which are composed of carbon and hydrogen atoms only. Figure 2-1: Ball-and-stick model of $$CX_4$$, Figure 2-2: Ball-and-stick models of some simple organic molecules. • These rules, used worldwide, are known as the IUPAC System of Nomenclature. Different compounds having the same molecular formula are called isomers , and the prevalence of organic isomers reflects the extraordinary versatility of carbon in forming strong bonds to itself and to other elements. Actually, atomic radii are so large relative to the lengths of chemical bonds that when a model of a molecule such as methyl chloride is constructed with atomic radii and bond lengths, both to scale, the bonds connecting the atoms are not clearly evident. Simple alkanes exist as a homologous series, in which adjacent members differ by a CH. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. However, as the size of the compound increases, it becomes difficult to portray it with the use of Lewis structure. There is a methyl group (rule 2) attached to the second carbon atom of the pentane chain. Models of ethene and methanal can be built with ball-and-stick models by using flexible couplings or bent sticks to form the double bonds (Figure 2-2), but the $$H-C-H$$ angles are inaccurate because they are $$109.5^\text{o}$$ rather than the observed $$117^\text{o}$$ to $$118^\text{o}$$. Unfortunately, structural formulas are difficult to type/write and take up a lot of space. For example, we can represent pentane (CH3CH2CH2CH2CH3) and isopentane [(CH3)2CHCH2CH3] as follows: Parentheses in condensed structural formulas indicate that the enclosed grouping of atoms is attached to the adjacent carbon atom. Alkyl groups are not independent molecules; they are parts of molecules that we consider as a unit to name compounds systematically. A systematic way of naming hydrocarbons and other organic compounds has been devised by the International Union of Pure and Applied Chemistry (IUPAC). 2.2: The Sizes and Shapes of Organic Molecules Last updated; Save as PDF Page ID 21954; 2-2A Bond Angles and Ball-and-Stick Models; 2-2B Bond Lengths and Space-Filling Models; Contributors and Attributions; The size and shape of molecules are as much a part of molecular structure as is the order in which the component atoms are bonded. The principle of homology gives organization to organic chemistry in much the same way that the periodic table gives organization to inorganic chemistry. Methyl groups (rule 2) are attached to the second and fifth carbon atoms. There are methyl and ethyl groups (rule 2), both attached to the fourth carbon atom (counting from the. Thus Table 2-1 shows that single $$C-C$$ bonds become progressively shorter as the number of multiple bonds or electronegative atoms attached to the carbons increases. Performance & security by Cloudflare, Please complete the security check to access. Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a homologous series. Much of the remainder of your study of organic chemistry will be taken up with learning about how the different functional groups tend to behave in organic reactions. Saturated, in this case, means that each carbon atom is bonded to four other atoms (hydrogen or carbon)—the most possible; there are no double or triple bonds in the molecules. Server 1 Server 2. You can number the parent chain from either direction as long as you are consistent; just don’t change directions before the structure is done. Unable to watch the video, please try another server . For this reason, ball-and-stick models or their equivalent are more generally useful than the space-filling models for visualizing structures and the positions of the atoms relative to one another. Carbon has four valence electrons and the electron configuration of carbon in group state is 1s2 2s2 2px1 2py1 2pz0. They are distinguished by the types of bonding between carbon atoms and the properties that result from that bonding. Interestingly, such compounds are much less stable and more reactive than otherwise similar molecules for which models can be constructed with straight sticks at tetrahedral angles. To describe functional groups and explain why they are useful in the study of organic chemistry. Ideally, a model should reflect not only the size and shape of the molecule it represents but also the flexibility of the molecule. Each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. Organic compounds, like inorganic compounds, obey all the natural laws. Alkanes have both common names and systematic names, specified by IUPAC. Because of their importance in understanding organic chemistry, functional groups have characteristic names that often carry over in the naming of individual compounds incorporating specific groups. It is well established that the normal carbon atom forms its four single bonds in compounds of the type $$CX_4$$ so that the four attached atoms lie at the corners of a regular tetrahedron. Missed the LibreFest? The rules enable us to not only name a compound from a given structure but also draw a structure from a given name. Then add the groups at their proper positions. Watch the recordings here on Youtube! Watch Next Video. We begin our study of organic chemistry with the hydrocarbons, the simplest organic compounds, which are composed of carbon and hydrogen atoms only. A 'family' of organic compounds with the same functional group is called a homologous series. Organic molecules strongly resist deformation forces that alter their valence angles from normal values. In the ball-and-stick type, the sticks usually come in various lengths to simulate different kinds of bonds; $$C-H$$ bonds typically are scaled to represent $$1.1 \: \text{A}$$, $$C-C$$ bonds to be $$1.54 \: \text{A}$$, and $$C=C$$ and $$C \equiv C$$ to be correspondingly shorter. In this chapter, we make a brief yet systematic study of some of organic compound families. Methane has a tetrahedral shape that chemists often portray with wedges indicating bonds coming out toward you and dashed lines indicating bonds that go back away from you. While the ball-and-stick models of molecules are very useful for visualizing the relative positions of the atoms in space, they are unsatisfactory whenever we also want to show how large the atoms are. Ethyne, $$C_2H_2$$, has been established experimentally to be a linear molecule; that is, the $$H-C-C$$ bond angle is $$180^\text{o}$$: This geometry also results with ball-and-stick models, if the triple bond is constructed of three flexible couplings or bent sticks as shown in Figure 2-2. This result led to a series of experiments in which a wide variety of organic compounds were made from inorganic starting materials. Thus, structural formulas identify the specific isomers by showing the order of attachment of the various atoms. The bond angles $$X-C-X$$ are $$109.5^\text{o}$$ and this value is the "normal" valence angle of carbon. $$^1$$The angstrom unit likely will be replaced eventually by the nanometer ($$1 \: \text{nm} = 10^{-9} \: \text{m} = 10 \: \text{A}$$).