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Introduction

There are two main styles for depicting acyclic structures and fragments, the "chain" (or "zig-zag") form [need better term?] and the "linear" form [need better term?]. Since it is possible to use the linear form only for relatively simple structures, the chain form is more generally useful. The linear form, however, remains acceptable for any structure to which it is suited. Most issues that apply to the chain form would also apply to the linear form. Other issues specific to the linear form are discussed separately.

Structures should be oriented horizontally

As stated in the general overview, all structures should be oriented horizontally when possible. This always is possible for the main chain in an acyclic structure.

RECOMMENDED	AVOID	AVOID

RECOMMENDED	AVOID	AVOID	AVOID	AVOID	AVOID

Principal groups should be toward the right

As stated in the general overview, any principal group(s), if present, should be oriented toward the right side of the structure. In a perfect application of this guideline, principal groups would be identified using the comprehensive hierarchical guidelines provided in other IUPAC recommendations [need to add references]. For most practical purposes, it is sufficient to treat "principal groups" and "acyclic heteroatoms" as synonymous. Thus, this rule could be oversimplified to say that any acyclic heteroatoms should be oriented toward the right side of the structure. In the absence of a formal functional group (according to the IUPAC recommendations), any branching chain should be considered instead

Since systematic numbering starts nearest the principal group(s), this guideline also implies that structures should be oriented so that systematic numbering increases from right to left within the structure.

RECOMMENDED	RECOMMENDED	RECOMMENDED	AVOID	AVOID	AVOID

However, esters of organic acids should be oriented with the acid group to the right of its parent, even if the ester derivative of the acid is physically larger.

RECOMMENDED	RECOMMENDED	AVOID	AVOID

Double bonds should be toward the left

In the absence of a different overriding concern, double bonds should be toward the left. [Is this right? Feels right to me, but seems a little unusual. On the other hand, it's sort of like the "rings to the left" rule]. If there are no double bonds, then triple bonds should be considered instead.

RECOMMENDED	RECOMMENDED	AVOID	AVOID

Branching double bonds should be upwards

A double bond that branches from the chain should preferentially be positioned above the chain rather than below it, even if that requires a change in the "zig-zag direction" of the chain. This is most commonly seen in ketones and carboxylic acids.

RECOMMENDED	RECOMMENDED	AVOID	AVOID

Initial bond should be upward

In the absence of a different overriding concern, the leftmost bond of a chain should be oriented upward when viewed from left to right

RECOMMENDED	RECOMMENDED	AVOID	AVOID