结构式

结构式

用元素符号和短线表示化合物(或单质)分子中原子的排列和结合方式的式子. 编辑本段甲烷的分子结构式

如甲烷的分子结构式可以表示为： H H ╲ ╱ C ╱ ╲

H H

结构式用—、＝、≡分别表示1、2、3对共用电子；用→表示1对

脱落酸结构式

配位电子，箭头符号左方是提供孤对电子的一方，右方是具有空轨道、接受电子的一方。 结构式可以在一定程度上反映真正的分子结构和性质，但不能表示空间构型，如甲烷分子是正四面体，而结构式所示的碳原子和四个氢原子却都在同一平面上。 编辑本段确定结构式的方法

确定一个化合物的结构是一件相当艰巨而有意义的工作．测定有机化合物的方法有化学方法和物理方法．化学方法是把分子打成“碎片”，然后再从它们的结构去推测原来分子是如何由“碎片”拼凑起来的．这是人类用宏观的手段以窥测微观的分子世界．50年代前只用化学方法确定结构确实是较困难的．例如，很出名的麻醉药东莨菪碱，是由植物曼陀丹中分离出来的一种生物碱，早在1892年就分离得到，并且确定其分子式为C17H21O4N．但它的结构式直到1951年才肯定下来．按照现在水平来看，这个结构并不太复杂．近年来，应用现代物理方法如X衍射、各种光谱法、核磁共振谱和质谱等，能够准确、迅速地确定有机化合物的结构，大大丰富了鉴定有机化合物的手段，明显地提高了确定结构的水平． 分子结构包括了分子的构造、构型和构象．构造是分子中原子成键的顺序和键性．以前叫做结构，根据国际纯粹和应用化学联合会的建议改为“构造”．表示化合物的化学式叫做构造式．

由于有机化合物中存在着同分异构现象，因此一个分子式可能代表两种或两种以上具有不同结构的物质．在这种情况下，知道了某一物质的分子式，常常可利用该物质的特殊性质，通过定性或定量实验来确定其结构式．

结构式不同而化学式相同不一定是同一种物质，其性质也往往不一样吗。比如各种有机物的同分异构体，化学式相同，但是结构式不一样，就显示出性质的差异。更不必说相同化学式的不同类物质，比如二甲醚和乙醇的分子式均为C2H6O,但其结构不同。 编辑本段结构式输入工具

随着计算机的普及，现代化学化工行业中有机物结构式的输入成为一大难题。现有的键盘、输入法中无法完成复杂结构式的录入。行业人员主要是通过专业的软件进行电脑输入： 1.化学语言编排软件

该软件较为简单，通过庞大的数据库收录了化学学科中经常用到的结构组成部分，用户可直接点击完成所需结构式、分子式等的录入，同时可将书写的结构式方便导入到常用格式的文档中。操作简单，在业界应用较为广泛。 2.化学金牌

利用word中宏的功能，完成简单分子式的输入，对于复杂结构式较难实现。 3.chemoffice

由国外推出的一款化学专业软件，可完成各种分子式、结构式等的输入，功能齐全。 结构简式

结构简式

就是结构式的简单表达式(通常只适用于以分子形式存在的纯

净物,如有机分子) :只要把碳氢单键省略掉即可，碳碳单键可以省略也可不省略，如，丙烷的结构简式为CH3CH2CH3，乙烯为CH2=CH2等。但是碳碳双键、三键一定不能省略。碳氧双键可省略，比如甲醛HCHO。

键线式

键线式（Skeletal formula），也称骨架式、拓扑式、折线简式，是在平面中表示分子结构的最常用的方法，在表示有机化合物的结构时尤其常用。用键线式表示的结构简明易懂，并且容易书写。 在用键线式表示复杂有机化合物的结构时有以下几点规则：

只用键线来表示碳架，两根单键之间或一根双键和一根单键之间的夹角为120?，一根单键和一根三键之间的夹角为180?，而分子中的碳氢键、碳原子及与碳原子相连的氢原子均省略，而其它杂原子及与杂原子相连的氢原子须保留。用这种方式表示的结构式为键线式。

1、画出分子骨架：画出除碳-氢键外的所有化学键。通常所有的氢原子及碳-氢键均省略不画，碳原子用相邻 的线的交点表示，一般情况下不用注明。单键用线段表

示，双键和叁键分别用平行的两条/三条线表示。对碳原子而言，单键/双键之间键角为120°，涉及三键的键角为180°。氢原子数可根据碳为四价的原则而相应地在碳上补充。例如，碳与两个基团相连时，补充两个氢；与三个基团相连时，补充一个氢，等等。

2、环形结构用相应边数的多边形来表示。 3、苯的结构可用两种方法表示。

4、官能团需要标明。标明时可以缩写（如-CN），也可以不缩写（如-C≡N）。 5、杂原子（非碳、氢原子）不得省略，并且其上连有的氢也一般不省略 6、某些基团或原子通常以特定的字母代替，常用的缩写见下，使用较不常用的缩写时通常会特殊说明：

X：卤素原子 M：金属或碱金属原子 R：烷基、烃基或任何基团 Me：甲基 Et：乙基 n-Pr：正丙基 i-Pr：异丙基 Bu：丁基 i-Bu：异丁基 s-Bu：仲丁基 t-Bu：叔丁基 Pn：戊基 Hx：己基 Hp：庚基 Cy：环己基 Ar：任何芳基 Bn：苄基 Bz：苯甲酰基 Ph：苯基 Tol：甲苯基 Xy：二甲苯基 Ac：乙酰基（多用于有机化学）、乙酸根（多用于无机化学）、锕 Bs：对溴苯磺酰基 Ns：对硝基苯磺酰基 Tf：三氟甲磺酰基 Ts：对甲苯磺酰基

7、反应物属于对映异构体之一时，需要将其立体化学特征表现出来。实线代表位于平面上的键，楔形实线表示向上伸出纸面的键，虚线代表向下伸出纸面的键，波形线代表键可以处于上述两种位置之一，即分子为外消旋混合物或键的立体特征不明。

8、氢键用虚线表示。

DRAWING ORGANIC MOLECULES

This page explains the various ways that organic molecules can be represented on paper or on screen - including molecular formulae, and various forms of structural formulae.

Molecular formulae

A molecular formula simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way

they are joined together.

For example, the molecular formula of butane is C4H10, and the molecular formula of ethanol is C2H6O.

Molecular formulae are very rarely used in organic chemistry, because they don't give any useful information about the bonding in the molecule. About the only place where you might come across them is in equations for the combustion of simple hydrocarbons, for example:

In cases like this, the bonding in the organic molecule isn't important.

Structural formulae

A structural formula shows how the various atoms are bonded. There are various ways of drawing this and you will need to be familiar with all of them. Displayed formulae

A displayed formula shows all the bonds in the molecule as

individual lines. You need to remember that each line represents a pair of shared electrons.

For example, this is a model of methane together with its displayed formula:

Notice that the way the methane is drawn bears no resemblance to the actual shape of the molecule. Methane isn't flat with 90° bond angles. This mismatch between what you draw and what the molecule actually looks like can lead to problems if you aren't careful.

For example, consider the simple molecule with the molecular formula CH2Cl2. You might think that there were two different ways of arranging these atoms if you drew a displayed formula.

The chlorines could be opposite each other or at right angles to each other. But these two structures are actually exactly the same. Look at how they appear as models.

One structure is in reality a simple rotation of the other one.

Note: This is all much easier to understand if you have actually got some models to play with. If your school or college hasn't given you the opportunity to play around with molecular models in the early stages of your organic chemistry course, you might consider getting hold of a cheap set. The models made by Molymod are both cheap and easy to use. An introductory organic set is more than adequate. Google molymod to find a supplier and more about them, or click on the picture or text link below to see a typical example from Amazon. (Don't click on the \button unless you really want to buy it!)

Share the cost with some friends, keep it in good condition and don't lose any bits, and resell it via eBay or Amazon at the end of your course.

Alternatively, get hold of some coloured Plasticene (or other children's modelling clay) and some used matches and make your own. It's cheaper, but more difficult to get the bond angles right.

Consider a slightly more complicated molecule, C2H5Cl. The displayed formula could be written as either of these:

But, again these are exactly the same. Look at the models.

The commonest way to draw structural formulae

For anything other than the most simple molecules, drawing a fully displayed formula is a bit of a bother - especially all the

carbon-hydrogen bonds. You can simplify the formula by writing, for example, CH3 or CH2 instead of showing all these bonds. So for example, ethanoic acid would be shown in a fully displayed form and a simplified form as:

You could even condense it further to CH3COOH, and would probably do this if you had to write a simple chemical equation involving ethanoic acid. You do, however, lose something by condensing the acid group in this way, because you can't immediately see how the bonding works.

You still have to be careful in drawing structures in this way. Remember from above that these two structures both represent

the same molecule:

The next three structures all represent butane.

All of these are just versions of four carbon atoms joined up in a line. The only difference is that there has been some rotation about some of the carbon-carbon bonds. You can see this in a couple of models.

Not one of the structural formulae accurately represents the shape of butane. The convention is that we draw it with all the carbon atoms in a straight line - as in the first of the structures above.

This is even more important when you start to have branched chains of carbon atoms. The following structures again all represent the same molecule - 2-methylbutane.

The two structures on the left are fairly obviously the same - all we've done is flip the molecule over. The other one isn't so

obvious until you look at the structure in detail. There are four carbons joined up in a row, with a CH3 group attached to the next-to-end one. That's exactly the same as the other two

structures. If you had a model, the only difference between these three diagrams is that you have rotated some of the bonds and turned the model around a bit.

To overcome this possible confusion, the convention is that you always look for the longest possible chain of carbon atoms, and then draw it horizontally. Anything else is simply hung off that chain.

It doesn't matter in the least whether you draw any side groups pointing up or down. All of the following represent exactly the same molecule.

If you made a model of one of them, you could turn it into any other one simply by rotating one or more of the carbon-carbon bonds.

How to draw structural formulae in 3-dimensions

There are occasions when it is important to be able to show the precise 3-D arrangement in parts of some molecules. To do this, the bonds are shown using conventional symbols:

For example, you might want to show the 3-D arrangement of the

groups around the carbon which has the -OH group in butan-2-ol. Butan-2-ol has the structural formula:

Using conventional bond notation, you could draw it as, for example:

The only difference between these is a slight rotation of the bond between the centre two carbon atoms. This is shown in the two models below. Look carefully at them - particularly at what has happened to the lone hydrogen atom. In the left-hand model, it is tucked behind the carbon atom. In the right-hand model, it is in the same plane. The change is very slight.

It doesn't matter in the least which of the two arrangements you draw. You could easily invent other ones as well. Choose one of them and get into the habit of drawing 3-dimensional structures that way. My own habit (used elsewhere on this site) is to draw two bonds going back into the paper and one coming out - as in the left-hand diagram above.

Notice that no attempt was made to show the whole molecule in 3-dimensions in the structural formula diagrams. The CH2CH3 group was left in a simple form. Keep diagrams simple - trying to show too much detail makes the whole thing amazingly difficult to understand! Skeletal formulae

In a skeletal formula, all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it.

For example, we've just been talking about butan-2-ol. The

normal structural formula and the skeletal formula look like this:

In a skeletal diagram of this sort

there is a carbon atom at each junction between bonds in a chain and at the end of each bond (unless there is something else there already - like the -OH group in the example);

? there are enough hydrogen atoms attached to each

carbon to make the total number of bonds on that carbon up to 4.

?

Beware! Diagrams of this sort take practice to interpret correctly - and may well not be acceptable to your examiners (see below). There are, however, some very common cases where they are frequently used. These cases involve rings of carbon atoms which are surprisingly awkward to draw tidily in a normal structural formula.

Cyclohexane, C6H12, is a ring of carbon atoms each with two hydrogens attached. This is what it looks like in both a structural formula and a skeletal formula.

And this is cyclohexene, which is similar but contains a double bond:

But the commonest of all is the benzene ring, C6H6, which has a special symbol of its own.

Note: Explaining exactly what this structure means needs more space than is available here. It is explained in full in two pages on the structure of benzene elsewhere in this site. It

would probably be better not to follow this link unless you are actively interested in benzene chemistry at the moment - it will lead you off into quite deep water!

Deciding which sort of formula to use

There's no easy, all-embracing answer to this problem. It

depends more than anything else on experience - a feeling that a particular way of writing a formula is best for the situation you are dealing with.

Don't worry about this - as you do more and more organic

chemistry, you will probably find it will come naturally. You'll get so used to writing formulae in reaction mechanisms, or for the structures for isomers, or in simple chemical equations, that you won't even think about it.

There are, however, a few guidelines that you should follow. What does your syllabus say?

Different examiners will have different preferences. Check first with your syllabus. If you've down-loaded a copy of your syllabus from your examiners' web site, it is easy to check what they say

they want. Use the \to search the organic section(s) of the syllabus for the word \

You should also check recent exam papers and (particulary) mark schemes to find out what sort of formula the examiners really prefer in given situations. You could also look at any support material published by your examiners.

Note: If you are working to a UK-based syllabus and haven't got a copy of that syllabus and recent exam papers, follow this link to find out how to get them.

What if you still aren't sure?

Draw the most detailed formula that you can fit into the space available. If in doubt, draw a fully displayed formula. You would never lose marks for giving too much detail.

Apart from the most trivial cases (for example, burning

hydrocarbons), never use a molecular formula. Always show the detail around the important part(s) of a molecule. For example, the important part of an ethene molecule is the carbon-carbon double bond - so write (at the very least) CH2=CH2 and not C2H4. Where a particular way of drawing a structure is important, this will always be pointed out where it arises

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